pressure experiment class 8

Force and Pressure | Chapter 2 | 8th Science - Student Activities | 8th Science : Chapter 2 : Force and Pressure

Chapter: 8th science : chapter 2 : force and pressure, student activities.

Fix a matrix of sharp pins on a wooden board in rows and columns. Take a big blown up balloon. Place it gently over the pins and place a small book on the top of the balloon. Will the balloon burst? Will the pins prick the balloon?

Aim :  To understand the effect of a force depends on the magnitude of the force and the area over which it acts.

Materials required  : Sharp pins, wooden board, balloon.

Procedure :

(i) Fix a matrix of sharp pins on a wooden board in rows and columns.

(ii) Take a big blown up balloon.

(iii) Place it gently over the pins.

(iv) Place a small book on the top of the balloon.

(v) Observe what happens?

(vi) Will the balloon burst? Will the pins prick the balloon?

Inference :

(i) The balloon will not burst. If you prick the balloon with a single pin it will burst. But this did not happen even though many more pins were pricking the balloon.

(ii) A single pin produces a large pressure over a small area. But, when a large number of pins prick a body, each pin exerts very little pressure on the balloon, as the applied force gets distributed over a large surface of the body. So, the balloon will not burst.

Conclusion:  We conclude that the effect of a force depends on the magnitude of the force and the area over which it acts.

More to know

Cooking in a place located at a higher altitude is difficult. Why? At a higher altitude, due to lack of atmospheric pressure the boiling point of a substance reduces. So, water boils even at 80 ° C. The thermal energy that is produced at this temperature is not sufficient enough for baking or cooking. So, cooking is difficult at higher altitude.

Take a conical flask and a well boiled egg, after removing its shell. Place the egg on the mouth of the flask. It will not enter the flask. Now take a piece of paper. Burn it and drop it inside the flask. Wait for a few seconds to burn fully. Now, keep the egg on the mouth of the flask. Wait for a few minutes. What do you observe?

When the paper is burning in the flask, the oxygen present in the air inside the conical flask is used up for its combustion. This reduces the pressure of the air in the flask. The air in the atmosphere tends to occupy the low pressure region in the flask. So, it rushes through the mouth of the flask, thus pushing the egg into the flask.

Aim :  To realise the atmospheric pressure.

Materials required:  Conical flask, boiled egg, piece of paper.

(i) Take a conical flask.

(ii) Take a well boiled egg, after removing its shell.

(iii) Place the egg on the mouth of the flask.

(iv) It will not enter the flask.

(v) Take a piece of paper.

(vi) Burn it and drop it inside the flask.

(vii) Wait for a few seconds, let it burnt fully.

(viii) Now keep the egg on the mouth of the flask.

(ix) Wait for a few minutes.

Observation:  The egg placed at the mouth of the flask gets compressed and it falls into the flask, due to the atmospheric pressure.

(i) When the paper is burning in the flask, the oxygen present in the air inside the conical flask is used up for its combustion. This reduces the pressure of the air in the flask. The air in the atmosphere tends to occupy the low pressure region near the flask.

(ii) So, it rushes through the mouth of the flask, thus pushing the egg into the flask. Eventually, the egg falls down to the bottom of the flask.

Take a plastic bottle. Punch three holes on its side in the same direction, but at different heights. Now pour some water into it and let it flow through the holes. Observe the flow of water. Water from the lowest hole comes out with the greatest force and the water from the topmost hole comes out with the least force.

Aim  : To understand that the pressure in a liquid varies with the depth of the point of observation in it.

Materials required :  Plastic bottle, Water

(i) Plastic bottle

(i) Take a plastic bottle.

(ii) Punch three holes on its sides in the same direction but at different heights.

(iii) Now pour some water into it and let it flow through the holes.

(iv) Observe the flow of water.

Observation:

i) The water comes out from all the holes with a different force and falls on the table at points that are at variable distances from the bottle.

(ii) Water from the lowest hole comes out with the greater force and falls at a point that is at the maximum distance from the bottle.

(iii) Water from the top most hole comes but with the least force and falls at the point that is at the minimum distance from the bottle.

Inference:  This shows that the pressure is a liquid varies with the depth of the point of observation on it.

Answer 2 :  Take a plastic bottle. Punch three holes on its side in the same direction, but at different heights. Now pour some water into it and let it flow through the holes. Observe the flow of water.

Inference :  The water comes out from all the holes with a different force and falls on the table at points that are at variable distances from the bottle. Water from the lowest hole comes out with the greatest force and falls at a point that is at the maximum distance from the bottle. Water from the topmost hole comes out with the least force and falls at the point that is at the minimum distance from the bottle.

Reason:  This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it.

This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it.

Take a glass tube that is open at both ends. Fix a rubber balloon at the lower end of the tube. Pour some water into the tube and observe the balloon. Now, pour some more water into the balloon and again observe the balloon. The balloon starts bulging outwards.

Aim  : To demonstrate that liquid exerts pressure at the bottom of its container depends on the height of the liquid column in it.

Materials required ;  Transparent glass tube, balloon, water.

(i) Take a glass tube that is open at both ends.

(ii) Fix a rubber balloon at the lower end of the tube.

(iii) Put some water into the tube and observe the balloon.

(iv) Now, pour some more water into the balloon and again observe the balloon.

Observation :  The balloon starts bulging outwards. The bulge increase with an increase in the height of the water column.

Inference:  The pressure exerted by a liquid at the bottom of a container depends on the height of the liquid column in it.

Take a plastic bottle. Punch three holes on its sides at the same height from its base. Now, pour some water into it and let it flow through the holes. Observe the flow of the water. Water comes out from all the holes with the same force and falls on the ground/ table, at the same distance from the bottle.

Aim  : To demonstrate that liquid exerts equal pressure at same depth.

Material required  : Plastic bottle, water.

(ii) Punch three holes on its sides at the same height from its base.

(iii) Now, pour some water into it and let it flow through the holes.

(iv) Observe the flow of the water.

Observation :  The water comes out from all the holes with the same force and falls on the ground / table, at the same distance from the bottle.

Inference :  This activity confirms that liquids exert the same pressure in all directions at a given depth in their container.

Why dams are made stronger and broader at the bottom than at the top? Why do scuba divers wear a special suit while they go into deep sea levels?

Take a rubber ball and fill it with water. Make tiny holes on its surface with a pin at different points. Press anywhere on the ball. What do you observe?

Aim :  To demonstrate pressure applied on one point of liquid transmits equally in all directions.

Materials required :  Rubber ball, water, pin.

(i) Take a rubber ball. Fill it with water.

(ii) Then take tiny holes on it with a pin at different points on its surface.

(iii) Press anywhere on the ball.

(iv) What do you observe?

Observation :  There are identical streams of water flowing in all directions from the holes.

Inference :  This is due to the phenomenon that the pressure, which is applied on the liquid, is equally transmitted in all directions.

Take some water in a beaker and spread a tissue paper on the surface of the water. Gently place the paper clip on the tissue paper. Observe what happens to the paper pin after some time.

After a few moments the tissue paper will submerge and the paper clip will make a small depression on the surface of the water. It will instantly begin to float on the surface, even though it is denser than water.

Aim :  To understand about surface tension property of liquid.

Materials required  : Glass beaker, water, paper clip, tissue paper.

(i) Take a paper clip. Take a beaker of water.

(ii) Take a tissue paper and spread it on the surface of the water. 

(iii) Gently, place the paper clip on the tissue paper.

(iv) Observe what happens to the paper pin after some time.

Observation :  After a few moments the tissue paper will submerge and the paper clip will make a small depression on the surface of the water. It will instantly begin to float on the surface, even though it is denser than water.

Inference :  This is due to the water molecules on the surface, which tend to contract themselves like the molecules of an elastic membrane. A force exists on them, which tends to minimize the surface area of water. The paper clip is balanced by the molecules on the water surface that is now behaving like a stretched elastic membrane. So, it does not submerge.

Take a small quantity of different kinds of liquid like coconut oil, honey, water and ghee etc. , Place one drop of each liquid on a separate glass plate. Now gently raise one end of the glass plate, so as to allow the liquid to slide down the smooth surface of the plate. Observe the speed of each liquid.

Aim :  To understand about the frictional force between the layers of liquid in motion.

Materials required  : Different kinds of liquid (coconut oil, honey, water, ghee), glass plates - 4 nos.

 (i) Take a small quantity of different kinds of liquid like coconut oil, honey, water  and ghee etc., in a cup.

(ii) Place one drop of each liquid on a separate glass plate.

(iii) Next, gently raise one end of the glass plate, one by one, so as to allow the liquid to slide down the smooth surface of the plate.

(iv) Observe the speed of each liquid.

Observation :  Each liquid moves with a different speed. Water flows faster than other liquids. Coconut oil flows with a moderate speed. Ghee flows very slowly.

Inference :  Between the layers of each liquid, in motion, there is a frictional force parallel to the layers of the liquid. This frictional force opposes the motion of the liquid layers while they are in motion.

Make two groups of students. Let them stand along a straight line, one behind the other, on a playground. Start the game of “tug of war” with a rope. Observe the movement of the students.

Who are the winners?

Aim: To understand the net effect of two forces acting on a body in opposite directions.

(iv) Make two group of students.

(v) Let them stand along a straight line, one behind the other, on a playground.

(vi) Start the game of ‘tug of war’ with a rope.

(vii) Observe the movement of the students.

(i) The rope will move in the direction of the team that applies more force.

(ii) If the two teams apply the same force, the rope will not move at all.

Fill two identical syringes with water. Connect them with a plastic tube. Press gently on one end of a piston. What do you observe?

Aim: To understand that the pressure exerted on a liquid at rest is transmitted equally to other portions of the liquid.

Materials required : Two identical syringes, water.

(i) Fill two identical syringes with water. Connect them with a plastic tube.

(ii) Press gently on one end of a piston.

(iii) What do you observe?

Observation: If one piston is pressed downward then the other piston will move up slightly, depending on the pressure given on the first piston.

Inference: The pressure exerted on a liquid at rest is transmitted equally to other portions of the liquid.

Aim: To understand that the rolling friction is less than the sliding friction.

Materials required: Book, table (rough surface), cylindrical pencils.

(i) Push or slide a book on a rough surface. It is difficult to push it. Isn’t it?

(ii) Now, keep some cylindrical pencils underneath the book.

(iii) Again, push it. It is easy to move the book. Why?

Observation: When you push the book, the pencils roll in the direction of the applied force. They prevent the contact of the book with the rough surface. Rolling pencils offer the least amount of friction.

Inference: So, it is easy to displace the book in comparison with sliding it on the table. This method is often used in moving heavy wood from one place to another. 

Aim  To understand the effect of the force of friction, which increases as the roughness the surface increases.

Materials required : Paper, glass, cotton cloth, wood, table, books, newspaper, writing pad, glass marbles in a bouch.

(i) Arrange some notebooks one over the other to form a platform, on a table.

(ii) Keep a wide scale, as a slide, such that one of its ends rests on the pile of books.

(iii) Take different kinds of materials like cotton cloth, plastic paper, newspaper, writing pad etc.

(iv) Place some glass marbles in a bowl placed on the table.

Experiment : First, keep a rectangular piece of paper near the end of the scale, which is in contact with the table. Now, release a glass marble from the top end of the scale such that it rolls down the scale. Allow the marble to roll over the piece of paper and finally, come to rest. Replace the rolling surface by placing the plastic sheet, wooden plank, cotton cloth.

Observation : Measure the distance travelled by the marble over the different objects using the meter scale.

Inference: In different objects each trial measure the distance travelled by the glass marble is tabulated. Tabulate the distance covered by the marble over different surfaces is different.

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• Chapter 11: Force And Pressure

NCERT Solutions for Class 8 Science Chapter 11 Force and Pressure

Ncert solutions for class 8 science chapter 11 – free pdf download.

* According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 7.

NCERT Solutions for Class 8 Science Chapter 11 Force and Pressure are provided here to help students to follow the concepts in an easy way. The topic of Force and Pressure is one of the key concepts in Science. It is very important to grasp every bit of this Chapter as they are basics for all students’ future endeavours. These  NCERT Solutions for Class 8 Science help them to attain perfection on the chapter by providing answers to a variety of questions such as MCQs, fill in the blanks, match the following, true or false and descriptive answer types questions.

Download Exclusively Curated Chapter Notes for Class 8 Science Chapter – 11 Force and Pressure

Download most important questions for class 8 science chapter – 11 force and pressure.

In order to score good marks in the Class 8 Science examination and to grasp the concepts involved in the chapter, students are advised to refer to the NCERT Solutions for Class 8 Science thoroughly.  Answers provided here will help students to get the knowledge on the concept of Force, magnitude and direction of force, change in its state of motion or a change in its shape, definition of pressure, and the exertion of pressure on liquid and gases. Access the Chapter’s NCERT Solutions for Class 8 of Science from the link provided below.

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Exercise Questions

1. Give two examples each of situations in which you push or pull to change the state of motion of objects.

Examples of situations in which you push or pull to change the state of motion of objects are

1. Pull situations

a) In order to open a drawer, we have to pull it. This action changes the state of motion of the drawer.

b) To draw water from the well, the rope is pulled. This action changes the state of motion of the bucket.

2. Push Situations

a) A football is pushed by the foot of a player. This action changes the state of motion of the ball.

b) In order to change the place of the heavy box from one room to another, we have to push it. This action changes the motion of the box.

2. Give two examples of situations in which applied force causes a change in the shape of an object.

The forces which change the shape of an object are as follows:

i) By pressing the clay between the hands, it deforms.

ii) The shape of the plastic bottle changes by squeezing it.

3. Fill in the blanks in the following statements.

(a) To draw water from a well, we have to __________ at the rope.

(b) A charged body __________ an uncharged body towards it.

(c) To move a loaded trolley, we have to __________ it.

(d) The north pole of a magnet __________the north pole of another magnet.

• To draw water from a well, we have to pull at the rope.
• A charged body attracts an uncharged body towards it.
• To move a loaded trolley, we have to pull or push it.
• The north pole of a magnet repels the north pole of another magnet.

4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information, fill up the gaps in the following statements using the following terms.

muscular, contact, non-contact, gravity, friction, shape, attraction

(a) To stretch the bow, the archer applies a force that causes a change in its __________.

(b) The force applied by the archer to stretch the bow is an example of __________ force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a __________ force.

(d) While the arrow moves towards its target, the forces acting on it are due to __________ and that due to __________ of air

(a) To stretch the bow, the archer applies a force that causes a change in its shape .

(b) The force applied by the archer to stretch the bow is an example of muscular force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.

(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

5. In the following situations, identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.

(b) Taking out paste from a toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

(d) An athlete making a high jump to clear the bar at a certain height

a) We make a muscular force to extract the juice of the lemon by squeezing it. As a result, the shape of the lemon gets changed.

b) To take out paste from the toothpaste tube, we use our muscular force. This muscular force acting on the toothpaste tube leads to a change in its shape.

c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.

d) The feet of the athlete exert muscular force on the ground, which pushes the ground. This allows them to jump over the bar. As a result, their state of motion gets changed.

6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

A blacksmith uses their muscular force while hammering a hot piece of iron. The muscular force changes the shape of the iron so that it can be given the desired shape.

7. An inflated balloon was pressed against a wall after it had been rubbed with a piece of synthetic cloth. It was found that the balloon stuck to the wall. What force might be responsible for the attraction between the balloon and the wall?

When an inflated balloon is rubbed with a piece of synthetic cloth, it becomes charged. A charged body attracts an uncharged body. When this charged balloon is pressed against a wall, it sticks to the wall. Thus, the electrostatic force acts between the charged balloon and the wall.

8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

For holding the bucket of water above the ground, we use muscular force. This muscular force acts against the force of gravity that pulls the bucket towards the ground. The two forces are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in the state of motion.

9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

The force of gravity is the one which acts on the rocket to pull it towards the ground, and the other one is the force of friction due to the earth’s atmosphere, which opposes its motion.

10. When we press the bulb of a dropper with its nozzle kept in water, the air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water

(b) gravity of the earth

(c) shape of rubber bulb

(d) atmospheric pressure

a)   Due to the atmospheric pressure, there is a rise of water in the dropper.

When all the air escapes from the nozzle, the atmospheric pressure, which is acting on the water, forces the water to fill up the nozzle of the dropper.

Class 8 Science NCERT Solutions for Chapter 11 Force and Pressure

The  NCERT Solutions for Class 8 will give students clarity regarding various aspects of this chapter in Science, which will consequently help them to solve questions from this chapter easily. Along with answers to the textbook questions, these solutions have exemplary problems , worksheets, important questions and HOTS.

In this Chapter 11, Force and Pressure, students will come across questions on applied force and changes in the object shape. Some questions, like the difference between force and pressure, appear prominently in the exams, which are explained in detail in these NCERT Solutions .

The other types of questions that are crucial are based on objects and force. Practising these types of questions in this chapter will ensure they have a good grasp of the subject. Here, the NCERT Solutions for Class 8 Science Chapter 11 PDF is also available, and students can access them offline by downloading it for free as well.

Subtopics of NCERT Solutions for Class 8 Science Chapter 11 Force and Pressure

• Force – A Push or a Pull
• Forces are Due to an Interaction
• Exploring Forces
• A Force Can Change the State of Motion
• Force Can Change the Shape of an Object
• Contact Forces
• Non-Contact Forces
• Pressure Exerted by Liquids and Gases
• Atmospheric Pressure

Students must study these NCERT Solutions meticulously to excel in the subject. These NCERT Solutions are prepared according to the latest syllabus of CBSE. Also, check the other NCERT Solutions of different classes and subjects at BYJU’S for free.

For more information, visit BYJU’S website or download BYJU’S – The Learning App and fall in love with learning.

Frequently Asked Questions on NCERT Solutions for Class 8 Science Chapter 11

What are the sub-topics covered under chapter 11 of ncert solutions for class 8 science, can students rely on the ncert solutions for class 8 science chapter 11 from byju’s, name the forces acting on a plastic bucket containing water held above ground level in your hand, which is studied in chapter 11 of ncert solutions for class 8 science..

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Pressure Exerted by Liquids and Gases

Last updated at April 16, 2024 by Teachoo

Pressure Exerted by Liquids

We know that all liquids have weight

Example-Water has weight

When this liquid gets filled inside a container,it exerts pressure because of weight of liquid

Example 1 Attaching Balloon to Test Tube

Suppose we attach balloon with test tube

We will fill the test tube with some water

It makes the balloon bulge a little

We put some more water in test tube

It makes the balloon bulge even more

This shows that water exerts pressure on balloon(more the water,more pressure is exerted on balloon

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Example 2 hole in water bottle.

We know that water flows through water Bottle

if there is hole in water Bottle,water comes out very fast like a fountain

This shows that water exert pressure on bottle.

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Gases exerting pressure.

We know that there are many gases like Nitrogen, Oxygen, Carbon Dioxide in the atmosphere

When these gases come in contact with a substance, there is collision of molecules of gas with molecules of substance

This causes air pressure

Example 1 - Inflating a balloon

When we inflate balloon,air comes inside the balloon because of pressure exerted by molecules of air on the balloon Due to this balloon inflates

We have to close its mouth

If we do not close its mouth,air escapes out

This shows air has pressure

Example 2 - Filling air in bicycle tyre

When air is filled into bicycle tyre by using a pump,the tyre tube gets hard because of pressure exerted by molecules of air on tyre tube of bicycle

If bicycle tyre gets punctured,air escapes very fast

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Class Notes

Free Class Notes & Study Material

Pressure Exerted by Gases

Last Updated on July 3, 2023 By Mrs Shilpi Nagpal

Question 1 Why makes a balloon get inflated when air is filled in it?

Question 2 Why a bicycle tube get inflated when air is filled in it?

All the gases exert pressure on the walls of the container.

(1) Air is a mixture of gases.

(2) All the gases are made up of tiny particles called molecules which move around quickly in all directions.

(3) The fast moving gas molecules collide with one another and with the walls of the container.

Air pressure arises due to the constant collision of the tiny molecules of the gases present in air with the walls of the container it is enclosed.

(4) If a certain mass of air is compressed into a smaller volume then the number of collisions of air molecules per unit area increases and hence the pressure exerted by air goes up.

(5) If the container of air has stretchable walls then the higher pressure exerted by air inflates the container.

Take a rubber balloon and fill air into it with mouth or by pump. On filling in air, the balloon gets inflated i.e. expands and become bigger in size. When we put a lot of air in the balloon then the number of gas molecules in the balloon increases too much. The large number of gas molecules cause too many collisions with the walls of the balloon from inside and create a high pressure air pressure. This high pressure produced by the gas molecules on the walls of balloon causes it to expand and get inflated.

When air is pumped into a bicycle tube by using a pump, the bicycle tube gets inflated due to the air pressure exerted by the collisions of gas molecules in air with the inner walls of the rubber tube. This air pressure in the bicycle tube makes the bicycle tyre feel hard.

About Mrs Shilpi Nagpal

Author of this website, Mrs. Shilpi Nagpal is MSc (Hons, Chemistry) and BSc (Hons, Chemistry) from Delhi University, B.Ed. (I. P. University) and has many years of experience in teaching. She has started this educational website with the mindset of spreading free education to everyone.

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Force and Pressure

1.1 INTRODUCTION A table can be moved from one place to another by either pushing it or pulling it

Similarly, you can open a door by either pushing or pulling it. When a ball is thrown with more force, it travels a longer distance. The shape of a bottle can change when it is squeezed. Also, the direction of a moving ball can change by striking it. Have you wondered how the shape of a bottle changes when squeezed? Or how can the direction of a moving ball change by kicking it in different ways? All the above activities can be associated with pushing or pulling . Therefore, whenever an object is moved, we can say that it has either been pushed or pulled. This push or pull is known as force. In other words, a body moves whenever a force is applied to it. Therefore, a body cannot move unless a force is applied. Apart from push or pull, force is any action that has the tendency to change the position, shape, or size of an object. Everyday actions such as pushing, pulling, stretching, lifting, squeezing, and twisting are also examples of force. Let us try to list some examples of everyday force and see if we can classify them as push or pull. Examples 1. Hitting a cricket ball with a bat 2. Opening a door 3. Plucking a flower 4. Flying a kite 5. Moving a wheel barrow 6. Hitting a tennis ball with a racquet 7. Taking a carrot out of the ground 8. Playing on a swing 9. Picking up a shopping bag 10. Squeezing a toothpaste tube

1.11 NEWTON’S FIRST LAW OF MOTION Let us suppose that a book is lying on a table. It is at rest. Can the book change its position by itself? Obviously, no. The book will not move by itself, unless you push or pull or lift it. Thus, the position of the book can change its position, only when some force is applied. A person riding a bicycle along a level road does not come to rest immediately after he stops pedaling, unless he applies the brakes. Here, on applying the brakes, the state of motion is changed to a state of rest. These examples contribute to Newton’s First law of motion. A body continues to be in the state of rest or of uniform motion along a straight line unless it is compelled by an external force to change its state. “Objects keep on doing what they are doing, unless they are acted upon by an unbalanced force”. The of Newton’s First law of motion introduces the concept of inertia and defines force. Therefore the first law of motion is also known as the law of inertia.

  ∴ 1   n e w t o n   ( N )   =   1   k g   m / s 2   1   d y n e   =   1   g   c m / s 2 Note : i) 1 newton is that much force which produces an acceleration of 1 m/s 2 in a body of mass 1 kg. ii) The force is said to be 1 dyne if it produces 1 cm/s 2 acceleration in a body of 1g mass. Relation between newton and dyne : 1 newton (N) = 1 kg m/s 2 = 1000 g × 100 cm/s 2 = 100000 g cm/s 2 = 10 5 gcm/s 2 = 10 5 dyne

Acceleration Due To Gravity The earth attracts every other body lying near or on its surface towards its centre. The origin of this force between the earth and a body is due to interaction between their masses. The force of attraction exerted by the earth on a body is called gravitational pull or gravity. We know that when force acts on a body, it produces acceleration. Therefore, a body under the effect of gravitational pull must accelerate. The acceleration produced in the motion of a body under the effect of gravity is called acceleration due to gravity. It is denoted by g. The value of acceleration due to gravity near the surface of the earth is 9.8ms -2 . The value of acceleration due to gravity does not depend upon, whether the body falling under gravity is a light one or a heavy one i.e., the value of g is independent of the mass of the body. If ‘g’ is acceleration due to gravity, ‘m’ is the mass of a body; then the gravitational force on the body is ‘mg’ and is called the weight of the body. weight = mass × acceleration due to gravity i.e. W = mg.

PRESSURE 1.16 INTRODUCTION

• Have you ever thought of these:
• How does a glider move in the air?
• Why a sudden outbreak of sand storm blew off the tops of the tents in desert.
• Why the paste comes out from the front end when it is pressed from the rear end?
• Why a camel moves faster than a horse in the desert?
• Why a sharp knife can cut easily in contrast to a blunt knife.
• How could we empty a cool drink bottle using a straw? The answers for these questions can be answered from this topic.

1.17 THRUST All bodies have a certain weight and a certain base area. When a body rests on a surface, it exerts a force equal to its weight, normal to its surface. This force is called thrust. Thrust is the total force or total weight acting normal to the surface. Weight of the body is the product of mass and acceleration due to gravity i.e., W = mg (where ‘g’ is acceleration due to gravity)

 Pressure  ( P ) =  Thrust  ( T )  Area  ( A ) Units: C.G.S. units: dyne/cm 2 or g.wt/cm 2 S.I. units: N/m 2 or kg.wt/m 2 Note: i) The S.I. unit N/m 2 is also known as Pascal. It is denoted by ‘Pa’. ii) The other units of pressure are bar, torr, cm of Hg and mm of Hg. iii) 1 atm = 10 5 Pascal = 10 5 N/m 2 = 10 6 dyne /cm 2 = 76 cm of Hg = 760 mm of Hg = 760 torr iv) Pressure on a body can be exerted from any direction. Hence, it has a no particular direction and is considered as a scalar quantity. Applications of Pressure i) Design of Wheels of different vehicles: Depending on the requirement, wheels of different vehicles are designed accordingly. Two illustrations are cited here under: Double wheel at rear end In general, the weight (force) is more at the rear end of the trucks, buses etc. This results in the development of greater pressure between the tyres and the road. If the pressure is more, the rate of damage is more and it may lead to accidents.

F 1 a 1 = F 2 a 2 = F 3 a 3 = F 4 a 4 = F 5 a 5

where a 2 , a 3 , a 4  and  a 5 are the areas of cross-section of pistons at B, C, D and E respectively. This indicates that pressure is transmitted equally in all directions as stated by Pascal’s law. Application of Pascal’s Law Pascal’s law is used to multiply force in machines such as the hydraulic press or Bramah press, hydraulic lift and hydraulic brake.

Principle of a Hydraulic Machine (Hydraulic Press or Bramah Press) Hydraulic press works on the principle of Pascal’s law. Construction : It consists of two cylindrical vessels X and Y of different cross sections. Let the cross sectional area of X be ‘A 1 ’ and that Y be ‘A 2 ’

=  Area of larger piston   Area of smaller piston  = π R 2 π r 2 = R 2 r 2

MA = (  Radius of larger piston  ) 2 (  Radius of smaller piston  ) 2

Note : i) Larger piston is generally referred as press plunger ii) Smaller piston is referred as pump plunger

Uses of Hydraulic Press 1. It is used for compressing the cotton bales and straw. 2. It is used for extracting oil from oil seeds. 3. It is used for punching holes in metals. 4. It is used for giving specific shapes to metal sheets. 5. It is used for servicing automobiles in service stations. 1.21 ATMOSPHERIC PRESSURE The atmosphere is a mixture of gases. Above the ground, the Earth’s atmosphere extends to a few hundred kilometers in height. Very close to the Earth’s surface, we find millions of gas molecules in every cubic centimeter. It is calculated that roughly around molecules per cubic centimeter are present. These molecules are all zipping around in different directions, colliding into each other and rebounding. They make an impact on each and every object by bombarding their surface resulting in pressure. The weight of the atmosphere also exerts force resulting in pressure. This pressure is known as Atmospheric pressure. The weight of thrust of air exerted on unit area of earth’s surface is called Atmospheric pressure. It is to be noted that the atmospheric pressure decrease as the altitude increases. The reason being that the number of molecules present at a higher altitude is less than the number of molecules present at the sea level. So, as the number of air molecules decreases with increases in altitude, the air pressure also decrease. Measurement of Atmospheric Pressure Atmospheric pressure is measured by using a barometer. It was invented by Torricelli in 1643. “The Mercury Barometer” Measurement of Atmospheric Pressure using Barometer Atmospheric pressure (P) can be obtained by a barometer, using the formula. P = h × d × g Where, ‘h’ is the height of liquid column, ‘d’ is the density of the barometeric liquid and ‘g’ is the acceleration due to gravity. If ‘h’ is expressed in meters, ‘d’ in Kg/m 3 and ‘g’ in m/s 2 , the pressure obtained by this formula is in terms of ‘Newton/m2 or ‘dyne/cm 2 . Pressure measured in this way is called Absolute pressure. In the formula ‘P = hdg’, the terms ‘d’ and ‘g’ for a given barometer are constant and the atmospheric pressure is proportional to the height of the mercury column. 1 atm is the pressure exerted by a vertical column of mercury of 76 cm (or 760 mm) height. ∴ 1 atm = 76 cm × 13.6 g cm -3 × 9.8 ms – 2 (using hdg for pressure exerted by a liquid) = 0.76 m ×13.6 × 10 3 kg m -3 × 9.8 ms – 2 = 101292.8 N m -2 =1.013×10 5 Pa Other units usually used in measurement of pressure are torr and bar 1 torr = 1 mm of Hg, 1 bar = 10 5 Pa 1.22 BUOYANCE A four – year child was questioned. ‘How do you wash your hands’ ? He promptly answered, ‘With the water form the tap’. A twelve – year boy was questioned: How do you get water in your tap ? He answered, ‘We have a storage tank the upstairs, which gets filled up when we switch on the motor’. Okay! This is how, in this modern age, we get water effortlessly. But in most of the villages the situation is entirely different. Even agricultural fields are watered by manual drawing, from wells, leave alone the domestic purpose. For their daily requirements, village people draw water from wells with the help of a bucket tied to a rope. When the bucket is released into the well, it gets filled up with water and then sinks to a certain depth in water. Now, try to pull out the rope and observe. While drawing water form a depth to the surface, we feel that the bucket of water is very light or almost weightless, but when we draw the water from the surface to the required height, we feel some heaviness or weight of water. From this act, it is clear that an object can be raised or lifted easily inside water. Heave you ever thought, what makes this raising or lifting easy ? This is due to the invisible force that is exerted by the liquid on the body in an upward direction. This upward force is called Buoyant force. From the above activity, it is very clear that some invisible force acts in upward direction when the body is immersed in water. This force makes the weight of the body (bucket) less than its actual weight. The loss in weight is caused due to the upward force acting on the body. It is called buoyant force and the property is called buoyancy. The upward force which is acting on a body when it is completely or partially immersed in a fluid is called up thrust or the buoyant force, and the phenomenon is known as buoyancy. Examples of Buoyant Force 1. When we lift a stone in air, we feel it heavier but inside the liquid we feel it to be lighter because water exerts and invisible force (upthrust) on the body, which decreases the weight of the body. That is why we feel it lighter inside the water than in the air. 2. When a swimmer jumps into water from a height, he finds himself being pushed towards the surface of the water without any effort on his part. 3. A stone which is dropped, falls down with an acceleration of 9.8 ms – 2 in the air. The same stone (density = 5 g cm -3 ) moves down in water with an initial acceleration of 7.8 ms – 2 due to upward buoyant force. Cause of Buoyant Force We know that a liquid contained in a vessel exerts pressure at all points and in all directions. According to Pascal’s law, the direction of pressure is normal (perpendicular) to the surface of the body.

The pressure at a given point depends on the depth (h) of the point inside the liquid. (P = hdg) Now, consider a body ABCD inside the liquid, as shown in the figure.

i) Side – wise Pressure : Consider two points ‘x’ and ‘y’ on the sides AD and BC respectively at the same depth as shown in the figure.

ii) Upward and Downward Pressure : The downward pressure acts on the upper surface of the body and an upward pressure acts on its lower surface. The lower face is at a greater depth than the upper face.

1.23 ARCHIMEDES’ PRINCIPLE Two thousand years ago there lived a Greek philosopher, Archimedes’. He was a scientist in the court of a king. The king had a doubt about the purity of gold with which his crown was made. So he asked Archimedes to invent a method of testing the purity of gold without spoiling the crown. One day, Archimedes noticed that when he had entered a fully filled bath tub, some water overflowed. Furthermore, he noticed that he felt considerably lighter in bath the tub. He reasoned that the loss in weight of his body in water must have some connection with the amount of water displaced by his body. He performed a simple experiment and came out with the relation between the apparent loss of weight and the weight of water displaced. This relation is called Archimedes’ principle.

Let us understand it form the following Take a spring balance and suspend 200 g stone to it. The weight of the stone in air is found to be 200 gf. Now, take an empty jar and fill it with water, till the water is one the verge of overflowing.

= W air  – W liquid  = 200 gf – 150 gf = 50 gf

The liquid displaced by the metal block is collected in a beaker and its weight is obtained. Let it be W gf. It is observed that the weight of the liquid displaced is equal to the loss of weight of the object. Weight of liquid displaced (W) = loss of weight W air  – W liquid  We also know, Loss of weight = Buoyant force ________ (2)     ∴ (1) = (2) Hence, we can say that buoyant force is equal to the weight of the liquid displaced. Thus, we can conclude that “Whenever a body is partly or wholly immersed in a fluid, it experiences an upward buoyant force which is equal to weight of the liquid displaced by the body”. This is known as Archimedes’ principle, which, in short, is Buoyant force = weight of liquid displaced Calculation of Buoyant Force We know that buoyant force is equal to the weight of the liquid displaced. Let the weight of the liquid displaced be ‘mg’. Buoyant force (mg) = V × d × g ( ∵ mass = volume × density) V liquid displaced  × d liquid  × g        _____ (1) If the body is inside the liquid, then the volume of the liquid displaced is equal to the volume of the body, Now the equation changes as Buoyant force = V body  × d liquid  × g    _____ (2) V liquid displaced  = V liquid 

i) Buoyant Force and Volume of Body Dip an iron block and a thermocol sheet of the same mass into the water. in which case do you feel the Buoyant force more? We know, buoyant force = V body  × d liquid  × g As they are dipped in the same liquid and at the same place, d liquid  and g are constants. ∴  Buoyant force  ∝ V body  Let us check their volumes. Volume of a body (V) = mass ( m ) density ( d ) As mass is same for both the bodies,  V ∝ 1   d As the density of thermocol is less than that of iron, volume of thermocol is more and therefore, it displaces more volume of water. Therefore, buoyant force is more on the thermocol sheet. Conclusion: If two bodies of the same mass are immersed in water, then the body with greater volume or less density experiences more buoyant force. In other words, the loss of weight is more for the body with greater volume. ii) Buoyant Force and Density of Liquid Take two beakers, one filled with water and the other with oil. Try to observe the loss of weight by immersing the body completely in water and oil separately. In which case do you feel more loss of weight ? The loss of weight can be found out, if buoyant force is known. We know, buoyant force = V body  × d liquid  × g As the same body is immersed at a given place, V body  and g are constant. ∴  Buoyant force  μ   d liquid  Let us check the densities of water and oil. As the density of oil is less than the density of water, buoyant force is more for water. Conclusion: If a body is immersed in water and oil, as the liquid with greater density produces more buoyant force, here, water with greater density produces more buoyant force. iii) Buoyant Force and Gravity Take a body of volume ‘V”, and immerse it in a liquid of density dliquid on the surface of the earth and on the surface of the moon respectively. In which case do you feel the buoyant force more? We know, buoyant force = V body  × d liquid  × g As the body and liquid are the same on both the surfaces, V body   and  d liquid  are constant. ∴  Buoyant force ∝ g Acceleration due to gravity on the earth is six times greater than the acceleration due to gravity on the moon. So, buoyant force is more on the earth than on the moon. Conclusion: A body in a given liquid experiences greater buoyant force at a place with greater value of ‘g’. Planets on which a body experiences highest and least Buoyant Force Of all the planets, the value of ‘g’ is least for Pluto and the planet with greater value of ‘g’ is Jupiter. So, buoyant force for a body in a liquid is the least on Pluto and greater on jupiter. The value of ‘g’ on Saturn is almost equal to that on earth. Hence, buoyant force experienced by a body in a given liquid on Saturn is the same as that on the earth.

1.24 FLOATATION Consider the following cases: Case-1: Stone in water A stone in water is acted upon by two forces as shown.

Case-3: Wood in Water What happens if the weight of the body is equal to the buoyant force ? If the weight of the body is equal to the buoyant force, the body neither sinks like the stone nor moves up like the balloon. But it floats.

Conditions 1. Sinking of a Body

2. Flotation of a Body

1.25 RELATIVE DENSITY Density Density of a substance is defined as its mass per unit volume,

i.e.,  Density  =  mass   volume 

The SI unit of density is kg m -3 . Its CGS unit is gcm -3 . Let us consider mercury of density 13.6 g cm -3 , alcohol of density 0.8 g cm -3 and aluminium of density 2.7 g cm -3 . If you compare the densities of mercury and aluminium, aluminium has less density than mercury, whereas if you compare aluminium and alcohol, aluminium is denser than alcohol. In this way, any substance can be heavier or lighter when compared to the other substance. To avoid this confusion, another term is devised, which is called relative density. In relative density, the density of a substance is compared to the density of water at 4 °C. The relative density of a substance is the ratio of the density of the substance to the density of water at 4°C. Relative density of a substance

=  Density of the substance   Density of water at  4 ° C _____ (1)

Note: i) Relative density is also known as specific gravity. ii) Relative density is the ratio of densities. So, it has no unit, but is a pure number. Significance of Relative Density Relative density of a substance tells us how many times a substance is denser than water. Example: If the relative density of a substance is ‘6’, it means that the substance is 6 times denser than water. Relative Density in terms of Weight of Body and Weight of Water displaced by the Body

We know, Relative density =  Density of substance   Density of water 

Relative Density = m substance  V substance  m water  V water 

If the volume of a substance is equal to the volume of the water, then, Relative density =  Mass of any volume of substance   Mass of water of an equal volume 

Multiplying the numerator and the denominator by ‘g’, then, we get, Relative density =  Mass of any volume of substance  × g  Mass of water of an equal volume  × g

∵   Relative   density   of   a   substance   ( or   body )

=  weight of any volume body in air   weight of equal volume of water  _____ (2)

We know that the weight of equal volume of water is equal to the loss of weight of the body in water. Loss of weight in water = Weight in air – Weight in water = Weight of water displaced _________ (3) Using (2) and (3), we can write

Relative density of a body =  weight of the body   loss of weight of the body in water  _________ (4)

Relative density of a body =  weight of the body in air   weight of the body in air – weight of the body in water  _________ (5)

Determination of Specific Gravity or Relative Density of Liquids Take a body which sinks in both the liquid and the water. Determine its weight in air, let it be W 1 . Now, determine its weight in the given water and liquid with a spring balance. Let it be W 2 and W 3 respectively. The body displaces the liquid equal to its volume(V) Loss of weight of the body in water = Weight of the body in air – Weight of the body in water = W 1 – W 2 Loss of weight of the body in the given liquid = weight of the body in air – weight of the body in liquid = W 1 – W 3

Relative density of liquid = w e i g h t   o f   ‘ V   ‘   v o l u m e   o f   l i q u i d w e i g h t   o f   ‘ V   ‘   v o l u m e   o f   w a t e r

But, we know that the weight of V volume of liquid = loss of weight of the body in liquid. Similarly, weight of ‘V’ volume of water = loss of weight of the body in water.

Relative density = L o s s   o f   w e i g h t   o f   t h e   b o d y   i n   l i q u i d   L o s s   o f   w e i g h t   o f   t h e   b o d y   i n   w a t e r

Relative density = W 1 – W 3 W 1 – W 2

By substituting the measured values of W 1 ,   W 2 , the relative density of the liquid can be determined.

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• Science CBSE
• Force and pressure
• Force & its types and pressure

9. Pressure exerted by liquids and gases

• Make a hole at the side of a plastic bottle.
• Fit a glass tube perfectly in the small hole, so there will be no water leak.
• Fit a balloon at the end of a glass tube.
• When water is poured on the glass tube, the balloon expands.

• Take a water bottle and make a hole at equal depth from the top of the bottle.
• Fill the bottle with water and note down the pressure.
• All the liquids exert the same pressure as shown in the below figure.

Living Science 2019 2020 Solutions for Class 8 Science Chapter 11 - Force And Pressure

• textbook solutions
• Force and Pressure

Living Science 2019 2020 Solutions for Class 8 Science Chapter 11 Force And Pressure are provided here with simple step-by-step explanations. These solutions for Force And Pressure are extremely popular among class 8 students for Science Force And Pressure Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science 2019 2020 Book of class 8 Science Chapter 11 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science 2019 2020 Solutions. All Living Science 2019 2020 Solutions for class 8 Science are prepared by experts and are 100% accurate.

Page No 146:

Question 1:.

A force can move a stationary object. If force is applied to a stationary object, does it always move?

No, it is not necessary that the stationary object will move always when a force is applied. If there is a friction force between the object and the ground then the friction force will oppose the motion of the object. Hence, the stationary object will move only if the applied force is greater than the friction force between the object and the ground. 

Question 2:

Two equal forces act on an object. Is this information enough to say which way the object will move? Give reasons.

No, this information is not enough. Two equal forces may or may not cause an object to move. If both the forces are acting on the same side of the object and are large enough to overcome the friction force between the object and the ground, then the object will move. If both the forces are acting on the object from the opposite directions then the object will not move as the net force on the object will become zero.  Hence, the direction of the force and value of the friction force must be defined in order to determine the movement of the object.

Question 3:

Name the force that acts on all bodies on the earth at all times.

Gravitational force is the force which acts all the time on all the bodies present on the earth. It's equal to the weight of a body and always directed towards the center of the earth. 

Question 4:

Name a force that always opposes motion.

The friction force is the force which always opposes the motion. Friction force acts due to irregularities and roughness of the surface.

Question 5:

Can a force act on an object even when it is not in contact with the object? Give examples.

Magnetic force is the force which can act on an object without making any contact with the objects. Example: Force exerted by a magnet on another magnet.

Page No 151:

A force acts perpendicular to a given surface. What is the quantity 'force per unit area' known as?

The quantity 'force per unit area' is known as 'pressure'. 

How is pressure related to the area over which a force acts?

The pressure is defined as the force per unit area. So, the pressure is indirectly proportional to the area over which the force acts. 

What is the force of 1 N acting over an area of 1 m 2 called?

Force per unit area is defined as the pressure. If 1 N of force is acting over an area of 1 m 2 , then it would be called unit pressure or pressure of 1 Pa.  

How does the pressure exerted by a liquid vary with  a. direction? b. depth?

(a) The pressure exerted by a liquid is the same in all the directions at the same height. (b) But it is directly proportional to the depth in the liquid. As the depth increases, the pressure exerted by the liquid increases too.

You are put into a room where the air pressure is very low. What effect will this have on your body?

The air present in the atmosphere exerts pressure on our body, but that pressure is compensated by our blood pressure which acts from inside out. If the air pressure of the surroundings will become very low then our blood vessels will burst out due to excess pressure of the blood inside our body.

Page No 152:

There is one force which is exerted by all matter on all other matter. Which force is this? (a) gravitational force (b) magnetic force (c) electrostatic force (d) frictional force

(a) Gravitational force According to Newton, every object in this universe attracts the other object with gravitational force.

Which of these is a contact force? (a) friction (b) magnetic force (c) gravitational force (d) electrostatic force

(a) Friction Frictional force is a contact force which resists the relative motion between two surfaces.

Page No 153:

Which of the following is true for the pressure exerted by a liquid? (a) pressure does not depend on depth. (b) pressure is only exerted in the downward direction. (c) Pressure is exerted both downwards and sideways but downwards pressure is greater than sideways pressure. (d) At the same depth, pressure is same in all directions.

(d) At the same depth, pressure is the same in all directions. Pressure exerted by a liquid increases with depth and acts in all directions, but it is the same at all points in a horizontal plane at a given depth in a  stationary liquid.

The SI unit of pressure is (a) kg/m 3 . (b) kg/m 2 . (c) pascal. (d) newton.

(c) Pascal The SI unit of pressure is the pascal (Pa).

Two forces A and B act on an object in opposite. A is bigger than B. The net force on the object is (a) A + B acting in the direction of A. (b) A − B acting in the direction of A. (c) A + B acting in the direction of B. (d) A − B acting in the direction of B.

(b) A - B acting in the direction of A When two forces are applied on an object in opposite directions, the net force on the object is the difference between the forces; this net force acts in the direction of the greater force.

Question 6:

Which of these can a force acting on an object not change? (a) direction of motion (b) state of rest (c) shape (d) mass

(d) Mass Mass is the amount of matter in an object; it cannot be changed by the application of a force.

Question 7:

The weight of an object is due to (a) gravitational force. (b) frictional force. (c) both gravitational and frictional force. (d) neither gravitational nor frictional force.

(a) gravitational force The weight of an object is nothing but the gravitational force with which it is attracted towards the centre of the Earth.

Question 8:

A force of 10 N acts on an area of 0.1 m 2 . The force is kept the same but the area is reduced to half. Which of the following is true? (a) The pressure does not change. (b) The pressure reduces to half. (c) The pressure increases by 1.5 times. (d) The pressure doubles.

(d) The pressure doubles. Pressure (P) = Force (F)/Area (A) = 10 N / 0.1 m 2 = 100 N/m 2 = 100 Pa Now, according to the question, New area, A' = A/2 and F' = F ∴ P' = F'/A' = 2F/A = 2P = 2 ×  100 = 200 Pa Thus, the pressure gets doubled.

All pushes and pulls are forces. True or false?

True. All pushes and pulls are forces and the direction in which an object is pushed or pulled is considered the direction of the force.

If a force acts on a body it will move in the direction in which the force acts. Ture or false?

False. Suppose you are driving a bicycle and you apply the brakes. Although the stopping force will be applied on the direction opposite to the direction of motion of the bicycle, the bicycle will move forward. Thus, the bicycle will move in the opposite direction of the applied force.

A force with magnitude A and another with magnitude B act on an object in the same direction. What is the net force acting on the object?

The net force acting on the object is A + B. If two forces are applied on an object in the same direction, then the net force acting on the object is a single force whose magnitude is the sum of the two forces.

A force with magnitude A and another smaller force with magnitude B act on an object in the opposite directions. What is the net force acting on the object?

If two forces are applied on an object in opposite directions, then the net force acting on the object is the difference between the two forces. Thus, in the given case, the net force acting on the object is A - B.

Does a force acting on a body always cause a change in its state of motion?

No. The net force of all forces determines the acceleration of the body.

Only the earth exerts gravitational force on all objects. True or false?

False. All matter in the universe exert gravitational force on other matter .

What measures the earth's gravitational pull on an object− its weight or mass?

An object’s weight is a measure of the Earth's gravitational force acting on it. Weight = Force of the Earth's gravity acting on an object              = Mass  ×  Acceleration due to gravity

This force acts from a distance and affects only objects made of certain metals such as iron. Name the force.

Magnetic force A magnet can influence a magnetic material, such as a piece of iron, even when they are not in contact.

Question 9:

Which type of force is exerted by an electrostatic charge?

Electrostatic force is exerted by an object with electrostatic or static charge.

Question 10:

Which force tends to slow down objects or prevent them from moving?

Friction is a force that slows down objects or prevents them from moving.

Question 11:

The larger the area over which a force acts, the __________ is the pressure.

The larger the area over which a force acts, the lesser  the pressure is.

Question 12:

Does pressure exerted by a liquid increase or decrease with depth?

Pressure exerted by a liquid increases with depth and acts in all directions.

Page No 154:

Question 13:.

Every square centimetre of your body experiences a force equal to the weight of 1 kg due to the atmospheric pressure. True or false?

True . Due to the atmospheric pressure of air, we experience a force equal to that exerted by a mass of 1 kg on every square centimetre of our body.

Question 14:

Atmospheric pressure increases with height. True or false?

False. Atmospheric pressure is caused by the weight of air molecules above the atmosphere. As we go to higher altitudes, the air becomes thinner and the atmospheric pressure decreases.

What two things fully describe a force?

Force is a vector quantity. It can be described by two things: its magnitude and the direction in which it acts.

If several forces act in different directions on a body, in which direction will the body move?

If several forces act on a body in different directions, the effect on the body will be due to the magnitude and direction of the net force acting on it. So, the body will move in the direction of the resultant force.

If an object is thrown up, it finally comes down. Why?

Gravitational force is the force with which the Earth pulls everything towards itself. So, if an object is thrown up, it slows down and then comes down because of the gravitational force of the Earth.

What is the difference between mass and weight?

Why can a magnet act from a distance?

Every magnetic substance is surrounded by its own invisible magnetic field, depending on its pole strength. A magnet can act from a distance because of the field of magnetic force.

What is pressure? What does pressure depend on?

Pressure is the force acting perpendicularly on a unit area of an object. Pressure = Force/Area Pressure depends on two factors: (i) Force applied (ii) Area over which the force acts

What is atmospheric pressure? What is it caused by?

Atmospheric pressure is the force per unit area exerted on the surface of the Earth by the atmosphere. Atmospheric pressure is caused by the weight of air molecules in the atmosphere.

Why is it difficult to cut vegetables with a blunt knife?

It is difficult to cut vegetables with a blunt knife because it has more area compared to a sharp knife, which means that the pressure exerted by the blunt knife is less than that of the sharp knife. In the case of blunt knife, even a good effort may go futile, as the pressure applied by its edge may not be sufficient to cut vegetables.

What is force? What are the four main effects of force?

Force: Force can be defined as an influence that tends to set a stationary body in motion or stop a body from moving. The SI unit of force is kg m/s² or newton. Effects of force: 1. It can make a stationary object move and stop. 2. It can change the speed of a moving object. 3. It can change the direction of a moving object. 4. It can change the shape or size of an object.

What is the difference between contact and non-contact forces? Explain with the help of one example of each.

What is friction? Explain with an example.

Friction is a resistance caused by a surface. It is a contact force that resists the relative motion between two surfaces. For example, when you stop pedalling a bicycle, it slows down and stops after some time. This is because of the frictional force.

Why are caterpillar tracks used in battle tanks instead of tyres?

Caterpillar tracks are used in battle tanks instead of tyres because they considerably increase the area of contact. The larger the area of contact, the smaller the effect of force. Thus, a caterpillar track helps a battle tank in distributing its weight more evenly over a large surface area than wheels can. This prevents it from sinking in areas where wheeled vehicles of the same weight can easily sink.

How does the pressure exerted by a liquid change with depth? Explain with a diagram.

When you press a rubber sucker on a smooth surface, it sticks to the surface. Explain why this happens?

When we press a rubber sucker on a smooth surface, we remove some of the air between the sucker and the smooth surface. This deficiency of air lets the air outside to exert pressure on the surface of the sucker. Force due to the pressure keeps the sucker stuck to the surface.

Two forces act on an object. Explain what the net force acting on the object will be in different situations.

If two forces act on an object in different directions, then the effect on the body will be due to the magnitude and direction of the net force acting on it. Thus, the body will move in the direction of the resultant force. We can have two situations for two forces: (i) If forces A and B are applied on an object in the same direction, then they will add up. (ii) If forces A and B are applied on an object in opposite directions, then the net force on the object will be the difference between these forces.

Explain the effects that a force acting on an object can have on the object.

Effects of force: 1. Force can make a stationary object move or change its position of rest. 2. Force can change the speed of a moving object. 3. Force can change the direction of a moving object. 4. Force can change the shape or size of an object.

Describe an experiment to show that air exerts pressure in all directions.

Take a tin and boil some water in it. When steam begins to come out, cork it and stop heating it further. Put the tin in a trough and pour some cold water mixed with ice on it. You will observe that the tin gets crushed and becomes irregular in shape. This is because the cold water condensed the steam inside the tin to water. This left no air inside the tin. Thus, the pressure inside the tin became much lesser than that on the outside. The air pressure from outside acted on the tin from all directions and crushed it. The above experiment shows that air exerts pressure in all directions.

If a body is moving with uniform speed in a particular direction on a perfectly smooth surface, then no force is acting on it. True or false? Explain.

False. If no external force is applied on the body, then there must be two equal and opposite forces acting on the body. Both these forces add up to zero. 1. Weight of the body, W = mg (In downward direction) 2. Reaction force, N = W (In upward direction) Because the surface is smooth, there is no frictional force.

If you use a stick to push an object, the muscular force acting on the object is a non-contact force since your body is not in contact with the object. True or false? Explain.

False. Contact force is the force in which a direct contact is required between two bodies. The contact may also be with the help of a stick or a piece of rope. If we use a stick to push an object, we make the contact with the help of stick. This push or force is caused by the action of the muscles in our body. This muscular force is a contact force.

Gravitational force exists between you and a building. Why are you not pulled towards the building?

The magnitude of gravitational force depends on the masses of two objects and the distance between them. Because the Earth has the greater mass, objects on the Earth are pulled towards it. Also, there exists gravitational force between us and any building. However, because of the smaller mass of the building as compared to the earth, this force is very small to experience. Therefore, we are not pulled towards the building.

Why do you think it is necessary to define a separate quantity called pressure? Why is defining only force not enough?

It is necessary to define pressure as a separate quantity in order to evaluate the force acting on a surface. To measure and describe the effect of a force acting on a surface, we need both the amount of force applied and the area over which it is applied. So, only force is not enough for this purpose. We need a quantity that includes both the above things, and this quantity is pressure, which is defined as the force exerted per unit area.

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Suppose you are going in a jet plane, and one of the windows breaks due to an accident. What problems do you think you will face?

If one of the windows in a jet plane breaks due to an accident, the plane may undergo explosive decompression. At such low pressure, we can have problems like nose bleeding, because the pressure exerted by the blood in our body will be much higher than the pressure outside. This can cause blood vessels to burst.

If you press a rubber sucker on a rough surface, it does not stick to the surface. Why?

If we press a rubber sucker on a rough surface, it will not stick to the surface because an airtight seal cannot be achieved between the surfaces. It is so because air rushes into the space between the rubber sucker and the rough surface.

The value of gravity on the moon is one-sixth of its value on the earth. So, the objects weigh less on the surface of the moon as compared to the surface of the earth. The weight which Pallavi couldn't lift on the earth, she can easily lift on the moon because it feels lighter on the surface of the moon. 

View NCERT Solutions for all chapters of Class 8

SCIENCE EXPERIMENTS FOR CLASS 8

Science experiments for class 8 – cbse syllabus.

Activity-6.1: Cumbustible and non-cumbustible substances Activity-6.2: Air is essential for burning Activity-6.3: Air is essential for burning Activity-6.4: Test for ignition Activity-6.5: Structure of a flame

Activity-15.1: Charging by rubbing Activity-15.2: Charging by rubbing on different materials Activity-15.3: Types of charges and their interaction Activity-15.4: Transfer of charge Activity-15.5: Prepare a short report on earth quakes Activity-15.6: Prepare a short report on devastation caused by tsunami in India

Kolb Labs emphasizes the need of experiential learning for kids. Kolb labs offers Science Lab As A Service to schools. We visit schools and perform science experiments for class 8 students within school campus as per CBSE syllabus. Schools need not to worry about setting up science lab and maintaining it. Instructors from Kolb Labs will bring the equipment, material required for each science experiment for class 8 to school and demonstrate the activities as per CBSE syllabus.Experiential learning helps students to learn fast and understand the concepts clearly. More importantly, practical demonstration of science experiments will make learning fun and exciting for 8th class students. Learning by doing will help students immensely to improve their thought process. Kolb Labs encourages to adopt experiential learning at the very young age itself to influence the thought process of young minds towards innovation and creativity.There are 102 Science experiments for Class 8 as per CBSE Syllabus. Kolb Labs will perform most of the science experiments using the real material. Kolb labs will also use other methods like images, videos, prototypes etc. Kolb labs will encourage class 8 students to perform activities hands-on.Kolb Labs also helps Class 8 students to prepare science working models and science projects to participate in science fairs. Kolb Labs also encourages 8th class students to come up with new science project ideas with knowledge gained through science activities performed as per CBSE syllabus.

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Force and Pressure Class 8 Important Questions and Answers

Important questions of Class 8 Science Chapter 11 Force and Pressure is given below. These important questions will help students while preparing for the exam.  Practising these important questions will analyse their performance and work on their weak points. Score well in exam of Class 8 Science by going through these important questions. Students of Class 8 can download important questions of Class 8 Science Chapter 11 Force and Pressure PDF by clicking the link provided below.

Important Questions of Class 8 Science Chapter 11 Force and Pressure

Here you can get  Class 8 Important Questions Science  based on NCERT Text book for Class 8. Science Class 8 Important Questions are very helpful to score high marks in board exams. Here we have covered Important Questions on  Force and Pressure  for Class 8 Science subject.

Very Short Answer Questions

1. How can we decide whether an object is moving faster than the other? Answer:  By their speed.

2. What is the distance moved by an object in unit time called? Answer:  Speed.

3. How can we change the speed and the direction of a moving body? Answer:  By applying force.

4. What is force? Answer:  A push or a pull on an object is called force.

5. What is the requirement for a force to come into play? Answer:  Interaction between two objects is required for a force to come into play.

6. What is the resultant force when two forces act in same direction? Answer:  Forces applied on an object in the same direction add to one another.

7. What will be the resultant force when two forces act in opposite directions on an object? Answer:  When two forces act in opposite directions on an object then the net force will be the difference between two forces.

8. What happens in a tug of war when two teams pull equally hard? Answer:  When two teams pull equally hard then the rope does not move in any direction.

9. Name the term used to express the strength of a force. Answer: Magnitude.

10. When does the net force become zero? Answer:  When two forces acting on an object in opposite directions are equal then the net force becomes zero.

11. A ball is at rest. Push it gently. Does the ball begin to move? Answer:  Yes, the ball begins to move.

12. What happens when we push again while the ball is moving? Answer:  When we push a moving ball, then its speed increases.

13. Place your palm in front of a moving ball. Does your palm apply any force on the ball? Answer:  Yes, our palm applies a force on the ball.

14. What happens to the speed of the ball when you place your palm in front of the moving ball? Answer:  The speed of the ball is decreased.

15. What are the two states of motion? Answer:  There are two states of motion: (i) The state of rest (ii) The state of motion.

16. Does the application of a force would always result in the change in the state of motion of an object? Answer:  No, it does not always change the state of motion of an object.

17. What is muscular force? Answer:  The force resulting due to the action of muscles is known as muscular force.

18. Give two examples of muscular force. Answer:  Bending of our body and kicking the ball.

19. Does the force can change the speed of a moving object? Answer:  Yes.

20. What effect does a force put on the shape of an object? Answer:  The force may cause change in shape of the object

21. Is muscular force a type of contact force? Answer:  Yes, muscular force is a type of contact force.

22. What is force of friction? Answer:  The force which acts on all moving objects in opposite direction to the motion of the body is called force of friction.

23. What kind of force is friction? Answer:  Force of friction is also a type of contact force.

24. Why is force of friction called contact force? Answer:  Since the force of friction arises due to contact between surfaces of moving body and other surfaces, so it is called a contact force.

25. Give two examples of contact forces. Answer:  (i) Muscular force (ii) Force of friction

26. What are non-contact forces? Answer:  The forces acting from a distance without making contact are called non-contact forces.

27. Give an example of a non-contact force. Answer:  Magnetic force.

28. What is magnetic force? Answer:  The force exerted by a magnet on any magnetic object is called magnetic force.

29. What are the interaction of poles of two magnets? Answer:  (i) Like poles repel each other. (ii) Unlike poles attract each other.

30. What do you mean by electrostatic force? Answer:  The force exerted by a charged body on another charged or non-charged body is called electrostatic force.

31. What kind of force is an electrostatic force? Answer:  It is non-contact force.

32. What is gravitational force? Answer:  The force by which earth or any other object attracts objects towards itself is called the gravitational force.

33. Is the gravity a property of earth only? Answer:  No, gravity is not a property of earth only.

34. Is the gravitational force a contact or non-contact force? Answer:  The gravitational force is non-contact force.

35. Name the force due to which every object falls on the earth. Answer:  Due to gravitational force.

36. What do you mean by pressure? Answer:  The force acting on a unit area of a surface is called pressure. Pressure = Force/Area

37. How can we increase the pressure by exerting same force? Answer:  To increase pressure we should exert the same force on a smaller area.

38. Do liquids and gases also exert pressure? Answer:  Yes, liquids and gases also exert pressure.

39. What is the site of the pressure exerted by a liquid on the container? Answer:  A liquid exerts pressure on the walls of the container.

40. Do gases also exert pressure on the walls of containers? Answer:  Yes, gases also exert pressure on the walls of the containers.

41. What is atmosphere? Answer:  The envelop of air around us is called atmosphere.

42. What is atmospheric pressure? Answer:  The pressure exerted by the air is known as atmospheric pressure.

43. Can you separate two hemispheres, if all the air is suck out from them? Answer:  There is no air inside the two hemispheres, so only outer surface is in contact of atmospheric pressure. This is because we cannot separate them.

Short Answer Type Questions

1: What is force? What is its unit?

Answer:  A push or a pull on an object is called a force. It arises due to the interaction between two objects. Force has magnitude as well as direction. It may change in the state of motion of an object or it may bring about a change in the shape of an object. The unit of force is Newton.

2: What is change in state of motion? What brings change in state of motion?

Answer:  A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion. Force may bring the change in the state of motion of an object.

3: What is the difference between contact forces and non-contact forces? 

Answer:  Forces which act only when there is physical contact between two interacting objects are known as Contact forces. Example: Muscular force. Whereas Forces which can act without physical contact between objects i.e. those that can act from a distance, are called non-contact forces or field forces. Example: Magnetic force.

4: Give two example each of the situation in which you apply force to change state of motion of an object and to change shape of an object?

Answer:  A goal keeper applies force for saving a goal. By his action the goal keeper tries to apply a force on the moving ball. This leads to change in state of motion of ball. Force is applied to the shape of a ball of dough when it is rolled to make a chapatti.

5: Why is it difficult to hold a school bag having a strap made of a thin and strong string?

Answer:  It is difficult to hold a school bag having a strap made of a thin and strong string because it apply the large pressure on the shoulders due to very small contact surface area. The pressure is inversely proportional to the surface area on which the force acts. Therefore, pressure decreases if surface area increases.

6: Why Porters wear turbans when they have to carry heavy loads on their heads?

Answer:  Porters wear turbans when they have to carry heavy loads on their heads, to increase the area of contact. This reduces the pressure on the head.

7: Force applied on an object may change its speed. How?

Answer:   force applied on an object may change its speed. If the force applied on the object is in the direction of its motion, the speed of the object increases. If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object

8: Why Lorries and trucks carrying heavy loads have 8 tyres instead of four?

Answer:  Lorries and trucks carrying heavy loads have 8 tyres instead of four. Also the tyres are broader because this increases the area of contact with the ground, thus reducing the pressure exerted on the ground.

9: Why the sucker sticks to the surface over which it is pressed?

Answer:  When we press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because pressure of atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure.

10: What is pressure? How is pressure related to the surface area on which it acts?

Answer:  Force per unit area is called pressure. Pressure is inversely proportional to the surface area on which it acts. Smaller the area, larger the pressure on the surface for the same force.

11: When we stop pedalling the bicycle, it slows down and gradually stops .Why?

Answer:  When we stop pedalling the bicycle, it slows down and gradually stops because of force of friction between the surface of tyres of cycle and the ground that brings moving bicycle to rest. The direction of force of friction is always opposite to the direction of motion.

12: What is frictional force? How it arises?

Answer:  The force acting against the relative motion of surfaces in contact is called frictional force or friction. Friction is an example of contact force. Force of friction arises due to contact between surfaces.

13: What is magnetic force? What happens when we bring like poles of two magnets closer?

Answer:   A magnet can exert force on another magnet without being in contact with it; this force is known as magnetic force. Like poles of two magnets repel each other. Unlike poles of two magnets attract each other.

14: What is gravitational force? Is gravity the property of earth alone?

Answer:   Every object exerts a force on every other object. This force is known as gravitational force. Gravity is not a property of the earth alone. Every object in the universe whether large or small exerts gravitational force on every other object.

15: A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket just after leaving of the launching pad

Answer:   The force acting on a rocket is:

• Frictional force due to air
• Force of gravity acting in downward direction.

16: What will be the effect of force while squeezing a piece of lemon between the fingers?  

Answer:  Agent exerting pressure: Fingers. Object: Lemon. Effect of force is observed: Lemon juice is coming out, change in shape of lemon.

17: What will be the effect of force while taking out toothpaste from toothpaste tube?

Answer:  Agent exerting pressure: Fingers. Object: Tooth paste tube Effect of force is observed: Tooth paste is coming out, change in shape of tooth paste.

18: A blacksmith is hammering a hot piece of copper for making a tool, how does the force due to affect the piece of copper? 

Answer:  The force due to hammering changes the shape of the hot piece of copper to make tools.

19: What are the effects of the force while making high jump by an athlete?

Answer:  Agent exerting pressure: An athlete. Object: Body Effect of force: Jumping

20: Differentiate between atmosphere and atmospheric pressure.

Answer:  Atmosphere is the envelope of air surrounding our earth but the atmospheric pressure is the pressure exerted by this air.

21: Differentiate between friction and contact force.

Answer:   Friction is the force responsible for changing the state of motion of objects but contact force is the forces which act only when there is physical contact between two interacting objects.

22: What is a force? Explain with the help of some examples.

Answer:  Force is a pull or push of the objects. The action like, pushing, pulling, picking, hitting, lifting, running and bending are the examples of force. Moving or stopping of a body, changing shape and direction of motion of objects are various actions which show force in play.

23: How do we feel force in our daily life?

Answer:  Various big or small actions make us feel the force. We hit or catch many objects in our daily life. We see that a moving ball stops on its own. The ball changes the direction of its motion when it is hit with a bat. We make lassi etc. by churning of curd. These are many actions which help us to feel that a force is exerted.

24: Explain that forces are due to an interaction between objects.

Answer:  Imagine, a man is standing behind a stationary car. The car does not move due to his presence.

Now allow the man to push the car, it means he applies a force on the car. The car may begin to move in the direction of the applied force. Note that the man has to push the car to make it move. This example shows that at least two objects must interact with each other for a force to come into play.

25: What happens when (i) Two forces are exerted in same direction? (ii) Two forces are exerted in opposite directions?

Answer:  (i) When two forces are exerted in same direction on an object, then the forces are added and action becomes easy. (ii) When two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces.

26: What are the two factors on which effect of force depends?

Answer:  There are following two factors on which the effect of force depends: (i) Magnitude:  The strength of force is usually expressed by its magnitude. (ii) Direction of force:  We have also to specify the direction of force in which it acts. If the direction or magnitude of force changes, its effect also changes.

27: What are the effects of force?

Answer:  A force changes or try to change the (i) Speed of a moving body. (ii) Direction of motion of a body. (iii) Shape of a body.

28: What are states of motion?

Answer:  An object can be in two positions, one is at rest or other in motion ; both are its states of motion. Any change in this position (rest or motion) can be called the change in the state of motion. The state of motion of an object is described by its speed and the direction of motion.

29: How can a force change the states of motion?

Answer:  There are two states of an object: (i) Rest (ii) Motion When a force is applied on a body then it can move. In the same way a force can change the direction of a moving object as well as its speed. A force can stop a moving object. If the force is applied in the direction of motion at an object, then its speed is increased. If the force acts in opposite direction, then the speed of the object is decreased.

30: What is the effect of force on the shape of an object?

Answer:  A force can change or try to change the shape of an object. When a force is applied on an object then change in shape takes place. It may be smaller or greater. At last we can say that the application of force on an object may change its shape.

31: Explain contact and non-contact forces.

Answer:  Contact forces: The forces which come into play only when two objects come in contact with each other are called contact forces. Muscular force and force of friction are the examples of contact forces. Non-contact forces: The forces which come into play without any contact of objects with one another are called non-contact forces. Electrostatic and magnetic forces are the examples of non-contact forces.

32: What is muscular force? Why is it called contact force?

Answer:  The force resulting due to the action of muscles is known as the muscular force. Muscular force is called contact force because it comes in action only when two bodies come in contact with each other.

33: What do you understand by the force of friction?

Answer:  The force which always acts on all the moving objects and whose direction is always opposite to the direction of motion is called force of friction. Since the force of friction arises due to contact between surfaces, it is also called contact force.

34: What is electrostatic force? Why is it called non-contact force?

Answer:  The force exerted by a charged body on another charged or uncharged body is called electrostatic force. This force comes into play even when the bodies are not in contact, so it is called non-contact force.

35: Explain force of gravity.

Answer:  Objects or the things that fall towards the earth because the earth pulls them. This force is called force of gravity or the force due to gravitation. This is also called just gravity. It is an attractive force. This force acts on all objects.

36: What is pressure? What happens to the pressure when area on which it is applied increases?

Answer:  The force acting on a unit area of a surface is called pressure. Pressure = Force/Area on which it acts. The pressure is inversely proportional to the area on which force is applied. As the area on which force is applied is increased, the pressure decreases. We can say that pressure increases with decrease in area.

37: We observe that the wheels of buses and trucks are heavier than the wheels of cars or scooters. Why?

Answer:  The buses and trucks are heavy and require to exert less pressure. We know that pressure decreases as area of contact increases, so the tyres are broadened so that they exert less pressure on earth and move easily. If they exert more pressure, they will sink in earth. The scooters and cars have less wide wheels because they need more pressure for gripping.

38: What is atmospheric pressure?

Answer:  The envelop of air around the earth is known as atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by the air is called atmospheric pressure.

39: If the area of your head is 15 cm × 15 cm, how much air (in weight) would you carry on your head?

Answer:  The weight of air column of the height of the atmosphere and area 15 cm × 15 cm is equal to the weight of an object of mass 225 kg (2250 N).  We have air inside our body, so we are not crushed under this weight and pressure of air is equal at both the sides.

Long Answer Type Questions

1: Explain contact and non-contact forces. Give two examples for each.

Answer:   Contact forces:  Forces which act only when there is physical contact between two interacting objects are known as Contact forces. Example: Muscular force: This is the force we can exert with our bodies by using our muscles, e.g: push, pull etc. Frictional force:  The force acting against the relative motion of surfaces in contact is called frictional force or friction.

Non-contact forces: Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces. Example: Magnetic force: Magnets exert forces of attraction or repulsion on other magnets Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

2: (a) How can friction be reduced? (b) How can it be increased? Give examples.

Answer:  We can reduce Friction

1. By using wheels and ball bearings. Use of wheels between surface moving over each other reduces friction. Ball bearings have small balls of steel between steel surfaces. Because of the balls the steel surfaces can easily moves over each other.

2. By making the rubbing surfaces smooth by polishing them.

3. By using a suitable lubricant, like oil (for light machinery) or grease (for heavy machinery). This helps because fluid friction is less than solid friction.

4. Friction due to air (air resistance) or water is reduced by using streamlined shapes in aeroplanes or ships. A streamlined shape is narrow in front and broader at the back. Birds and aquatic animals have streamlined shapes which held them in flying or swimming.

We can increase Friction by the following ways:

1. Sand and gravel is strewn on slippery ground during the rainy season to increase friction. It is then easier to walk on the ground.

2. By making the moving surfaces rough, e.g. tyres have designs and patterns with grooves on the surface to increase resistance with the road. This prevents slipping of the tyres on a wet road.

3. To increase friction, spikes are provided in the soles of shoes used by players and athletes.

3: Do liquid and gases exert pressure on the walls of container in all direction? Give example to justify your statement. 

Answer:  Liquid and gases exerts pressure on the walls of container. For example, If we take a plastic bottle and drill four holes around near the bottom of bottle at the same height. After filling the water in that bottle, we observe that water comes out of the holes and falls at the same distance. This shows that liquid exerts pressure on the walls of container in all directions. Similarly, we are not able to inflate a balloon which has holes because air inside balloon exerts pressure in all direction. Hence we can say that gases exert pressure on the walls of container in all directions.

4: Why is it easy to push a nail into a wooden plank by the pointed end? 

Answer:   It is easy to push a nail into a wooden plank by pointed end because the smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail is much smaller than that of its head. The same force therefore produces a pressure sufficient to push the pointed end of the nail into the wooden plank.

5: Read Table and try to identify the action as push or pull.

Answer:  Table Identifying Actions as Push or Pull

6: Read Table and complete it.

Answer:  Table Studying the Effect of Force on Objects

  7: What are the various effects of force on different objects?

Answer:  The various effects of force are: (i) A force can make an object move from rest. (ii) It can change the speed of a moving object. (iii) It can bring about a change in the shape of an object. (iv) It can change the direction of motion of an object. (v) It can cause some or all of these effects.

8: What are contact forces? State different types of contact forces.

Answer:  The forces which come into play only when two objects come in contact with each other are called contact forces. Some contact forces are: (i) Muscular force: The forces resulting due the action of muscles are known as muscular forces. Muscular force is a contact force because it comes into play when two object, come in contact with each other. (ii) Force of friction: When a body is moving, then a force equal and opposite to the direction of motion is exerted on that moving body. This force is called force of friction. It is also a contact force because it is exerted when two surfaces come in contact with each other.

9: What are non-contact forces? Explain different types of non-contact forces.

Answer:  The forces which can be exerted from a distance, without establishing a contact are called non-contact forces. Some non-contact forces are:

(i) Magnetic force:  The force exerted by a magnet on other magnet or some other magnetic substance like iron is called magnetic force. Like poles of a magnet repel each other and unlike poles of a magnet attract each other without contact. So it is called non-contact force.

(ii) Electrostatic force:  The force exerted by a charged body on other charged or uncharged body is called electrostatic force. Electrostatic force also acts without making a direct contact with other charged or uncharged body. So it is also a non-contact force.

(iii) Force of gravity : Earth pulls every thing or body towards it. The force of attraction exerted by earth on any object is called force of gravity. This is also a non-contact force as it acts from a distance.

10: What is force? Name different types of forces.

Answer:  A push or pull on an object is called force. There are following types of forces: (i) Muscular force (ii) Force of friction (iii) Magnetic force (iv) Force of gravity (v) Electrostatic force

11: Prove that the force of friction depends on the nature of the two surfaces in contact.

Answer:  Collect the following things: A thick book, nylon cloth, gunny cloth, plastic sheet, jute cloth and sand paper. Place the book on each of the materials and slide it on the floor one by one. If your book slides off the plastic sheet or nylon cloth, use adhesive tape to stick it firmly on the surface of the book.

You will observe that the different materials offer different amounts of resistances to sliding. This activity shows that force of friction depends on the nature of the surface in contact. In general, smooth surface offers lesser friction than rough surface.

12: Prove that the pressure exerted by water at the bottom of the container depends on the height of its column.

Answer:  Take a transparent glass tube or plastic pipe. Also take piece of thin sheet of a good quality rubber. Stretch the rubber sheet tightly over one end of pipe. Hold the pipe at the middle, keeping it in a vertical position. Pour some water in the pipe. Note the height of the water column in the pipe. Pour some more water. Observe, the bulge in rubber sheet and height of water column in the pipe. Repeat this process a few more times. You observe that as the height of water column increases the bulge in the rubber sheet also increases.

13: Show that a liquid exerts pressure on the walls of the container.

Answer:  Take a plastic bottle. Fix a cylindrical glass tube a few cm long near its bottom. You can do so by slightly heating one end of the glass tube and then quickly inserting it near the bottom of the bottle. Make sure that water does not leak from the joint. If there is any leakage seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet. Now fill the bottle up to half with water. We observe the bulge in the rubber sheet. Pour some more water in the bottle. We see more bulge in rubber sheet. This activity indicates that water exerts pressure on the walls of the container.

14: Explain that liquids exert equal pressure at the same depth.

Answer:  Take an empty plastic bottle. Drill four holes all around near the bottom of the bottle. Make sure that all the holes are at the same height from the bottom. Now fill the bottle with water. We observe that different streams of water coming out of the holes fall at the same distance from the bottle. This observation indicates that liquids exert equal pressure at the same depth.

15: What experiment was performed to prove that air has pressure?

Answer: Otto von Guericke, a German Scientist invented a pump in 17th century to extract air out of a vessel. He demonstrated the force of the air pressure. He joined two hollow metallic hemispheres of 51 cm diameter each and pumped air out of them. Then he employed eight horses on each hemisphere to pull them apart. So great is the force of air pressure that the hemispheres could not be pulled apart.

NCERT Solutions for Class 6, 7, 8, 9, 10, 11 and 12

NCERT Exemplar Class 8 Science Chapter 11 Force and Pressure

June 20, 2022 by Sastry CBSE

NCERT Exemplar Class 8 Science Chapter 11 Force and Pressure are part of NCERT Exemplar Class 8 Science . Here we have given NCERT Exemplar Class 8 Science Chapter 11 Force and Pressure.

Multiple Choice Questions

Question. 3 During dry weather, while combing hair, sometimes we experience hair flying apart. The force responsible for this is (a) force of gravity (b) electrostatic force (c) force of frictio’n . (d) magnetic force Answer. (b) The electrostatic force is responsible for it, since on combing the hair, the comb and hair get oppositely charged.

Question. 5 Two objects repel each other. This repulsion could be due to the (a) frictional force only (b) electrostatic force only (c) magnetic force only (d) either a magnetic or an electrostatic force Answer. (d) The reason for repulsions may be either an electrostatic (in case of similar charges) or a magnetic (in case of similar pole_s) force.

Question. 6 Which one of the following forces is a contact force? (a) Force of gravity (b) Force of friction (c) Magnetic force (d) Electrostatic force Answer. (b) Force of friction is a contact force. It always acts when the bodies are in contact.

Very Short Answer Type Questions

Question. 9 A ball of dough Is rolled into a flat chapatti. Name the force exerted to change the shape of the dough. Answer. The shape of dough is changed due to the muscular force applied by the hand.

Question. 10 Where do we apply a force while walking? Answer. We apply a force on ground while walking and ground applies reaction force on our foot due to which we are able to move forward.

Question. 11 A girl is pushing a box towards East direction. In which direction should her friend push the box SQ that it moves faster in the same direction? Answer. Her friend should push the box towards East direction, so that it will start moving more fast towards East because the magnitude of force increases.

Question. 13 During dry weather, clothes made of synthetic fibre often stick to the skin. Which type of force is responsible for this phenomenon? Answer. The electrostatic force starts working between the cloth made of synthetic fibre and sticks to skin.

Question. 14 While sieving grains, small pieces fall down. Which force pulls them down? Answer. It is the force of gravity which is responsible for the grains to fall down.

Question. 15 Does the force of gravity act on dust particles? Answer. Yes, force of gravity acts on the dust particles.

Question. 16 A gas filled balloon moves up. Is the upward force acting on it larger or smaller than the force of gravity? Answer. The upward force will be greater than the force of gravity.

Question. 17 Does the force of gravitation exist between two astronauts in space? Answer. Yes, there will be gravitational force between the astronauts because every object in universe, whether small or large, exerts a force on every other object, it is the universal law of gravitation.

Short Answer Type Questions

Question. 18 A chapatti maker is a machine which converts balls of dough into chapatties. What effect of force comes into play in this process? Answer. The force on unit area is called pressure, works on the chapaties. This is the pressure which works on the dough balls and make them chapatties with the help of machine.

Question. 21 Two thermocol balls held close to each other but move away from each other, when they are released. Name the force which might be responsible for this phenomenon. Explain. Answer. This is electrostatic force which is created due to the rubbing and since, same charges are induced on two balls, so they move away from each other.

Question. 22 Fruits detached from a tree fall down due to the force of gravity. We. know, that a force arises due to the interaction between two objects. Name the objects interacting in this case. Answer. The interacting objects in this case are: earth and fruits. Earth applies force of gravity on fruit towards its centre. So, fruit falls down.

Question. 23 A man is pushing a cart down a slope. Suddenly the cart starts moving faster and he wants to sloyy it down. What should he do? Answer. Man can do following things: (i) He can start pulling the cart instead of pushing it in order to balance the downward force due to gravity. (ii) He can go the other side by moving himself very fast in the direction of motion and try to slow down the speed of cart by giving an opposite force to the moving cart.

Long Answer Type Questions

Question. 25 An archer shoots an arrow in the air horizontally. However, after moving some distance, the arrow falls to the ground. Name the initial force that sets the arrow in motion. Explain why the arrow ultimately falls down? Answer. The archer shoots an arrow by applying muscular force to stretches the string of the bow. When the string is released, it regains its original position that provides the initial force to set the arrow in motion horizontally. The force of gravity that acts on the arrow in the downward direction and hence, the arrow ultimately falls to the ground.

Question. 28 Two women are of the same weight. One wears sandals with pointed heels while the other wears sandals with flat soles. Which one would feel more comfortable while walking on a sandy beach? Give reasons for your answer. Answer. While walking on a sandy surface, one needs the footwears of larger area so that the pressure exerted on the ground is minimum. So, in this case, the woman having the sandals with pointed heels will be less comfortable in walking while the other woman wears sandals with flat soles feels more comfortable while walking on sandy beach.

Question. 29 It is much easier to burst an inflated balloon with a needle than by a finger. Explain. Answer. Because needle tip has very less area of cross-section in comparison to that of our finger and we know that pressure exerted by a force is inversely proportional to the area where it has been applied, so pressure exerted will be more by the needle tip than the finger. –

NCERT Exemplar Class 8 Science Solutions

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials : Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

NCERT Exemplar Solutions NCERT Exemplar Maths NCERT Exemplar Science

We hope the NCERT Exemplar Class 8 Science Chapter 11 Force and Pressure help you. If you have any query regarding NCERT Exemplar Class 8 Science Chapter 11 Force and Pressure, drop a comment below and we will get back to you at the earliest.

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20 Best Air Pressure Science Experiments / Science Fair Ideas

• November 3, 2022
• Science Experiments

We have put together a list of Air Pressure Science Experiments that is perfect for kids to try at home or to demonstrate their knowledge in a science fair .

These Air Pressure Science Experiments are a sure shot way of fun learning, experimenting, and exploring the fascinating forces of air pressure. These experiments can be conducted anywhere at home, playgrounds or outdoors .

Air Pressure Science Experiments

Before we step into our amazing experiments , let us learn a little about Air Pressure and its properties in words.

Air Pressure is the pressure created on the air molecules exerted by the air pressing down to the earth.

Generally, the air pressure is determined by three factors: Temperature , Moisture , and Altitude. Commonly air pressure is measured using a mercury barometer .

Here are the science activities or experiments to demonstrate Air Pressure to children.

1. Drinks Dispenser Science Activity

We usually observe that Kids are curious about dispensing liquids out of the bottles while adults do the same at parties or in the home. Why don’t we let them prepare their own drinks dispenser! Check out whether our experiment helps our kids in making drinks dispensers in no time and with fewer materials.

Click Drinks Dispenser Science Activity to get all the details before you start investigating.

2. Hot Air Cold Air Science Activity

Performing this science activity is a perfect opportunity for the kids to understand the concept of cold and hot air. In addition, they will get the chance to talk about what is actually happening with the water and air together and to explore the reason or science behind it.

Click on Hot Air Cold Air Science Activity

3. Egg in the bottle Air Pressure Science Experiment

An interesting and entertaining science activity with eggs to understand the differences in air pressure!! Kids, get ready to perform this activity and amaze your friends and family.

For more details about the cool science experiment on air pressure: Browse Egg in the Bottle Air Pressure Science Experiment

4. Oxygen and Air Pressure Experiment

We cannot see the air but we always feel the pressure of the air around us on everything!! Great experiment with a better explanation, demonstration, and appropriate result. It works effectively to start homeschooling with your kids as the little magical trick explains clearly how air pressure works with oxygen.

It is better to browse the experiment once before you start the experiment: Click on Oxygen and Air Pressure Experiment

5. Newton’s Law of Motion Air Pressure Experiment

This is a fun science experiment for preschoolers and kindergartens to explore Air science in a wonderful way!! Kids can perform this cool activity on their own and be amazed to see the magical results of the experiment.

Grab the materials here and get ready to explore air pressure: Newton’s Law of Motion Air Pressure Experiment

6. Balloon in a Bottle : Air Pressure Experiment

This is a simple experiment that shows how Air Pressure works.

Objective: Kids learn how air and air pressure are able to expand a balloon and can have a great demonstration of air pressure.

For more details about the balloon in a bottle: air pressure Browse Balloon in a Bottle: Air Pressure Experiment

7. Balloon and Pin Experiment

Here is an interesting experiment that shows you can make an un-poppable balloon.

A sharp object is a bad friend to an inflated balloon because it lets the balloon pop upon contact! But a pack of the same sharp object becomes a great friend to the same balloon.

Are you interested in learning about what the magical science around balloons and pins? Let’s dive into the Balloon and Pin Experiment (Air Pressure Experiment for Kids)

8. How to Put a Skewer Through a Balloon: Science Fair Project

Do you think an inflated balloon pops out when you insert a skewer into it, as always? Of course, Yes!

But there is a simple trick to insert a sharp-ended skewer into the balloon without blasting it. Let’s learn about this Non-popping balloon experiment.

Though it appears easy, you may not succeed in one or two attempts.

Let’s try this interesting experiment How to Put a Skewer Through a Balloon

9. Crushing Can Experiment: Effect of Atmospheric Pressure

You may be used to crushing cans using foot or hand. Have you crushed it using an implosion? Today we are going to explore the effect of Atmospheric Pressure with the ‘Crushing Can Experiment’.

Let’s work on this interesting experiment Crushing Can Experiment: Effect of Atmospheric Pressure

10. Drip Drop Bottle-Water Bottle Pressure Experiment

Are you aware of the magic water bottles? We are going to perform a very simple ‘Drip Drop Water Bottle Pressure Experiment’, which helps us to make the ‘Magic Water Bottle’.

Let’s check it out by clicking Drip Drop Bottle-Water Bottle Pressure Experiment

11. How to Build a Fast Balloon Powered Car

This one is an awesome engineering project, ‘Build a Balloon Powered Car’. In this project, we are going to learn about Newton’s Third Law and how it is applied to design propulsion vehicles such as cars or rockets, etc.

Let’s try this by clicking How to Build a Fast Balloon Powered Car

12. How To Make a Balloon Hovercraft

Hovercrafts might be old-fashioned means of transport, but they offer a ton of fun and education to children as a science fair project.

Today, we will learn about creating a ‘homemade version of hovercraft’ using just an old CD and a balloon.

Trying this by visiting How To Make a Balloon Hovercraft

13. Air Pressure Hands-on Experiments for Toddlers and Pre-Schoolers

It is a little tricky to explain the concept of air pressure to the kids who are preschoolers and homeschoolers!!

Click the link below to find the two experiments back to back demonstrating air pressure in a simple and neat way.

To know the instructions and materials required to perform these experiments: Click here, Air Pressure Hands-on Experiments for Toddlers and Pre-Schoolers

14. How does a paper towel stay dry Science Experiment?

Extremely easy activity to perform by your young kids. If you are a teacher or a parent, this simple science activity is perfect to introduce air pressure to the younger children in an entertaining way.

Get the details of the simple and fun activity that demonstrates air pressure here: How does a paper towel stay dry Science Experiment?

15. Air Pressure Experiment – Bernoulli Principle

A perfect experiment to understand Bernoulli Principle in an easy and neat way. Just an empty squash bottle is enough to investigate this experiment in simple steps. Wondered!? Browse the experiment to make your children WOW by the magical results it gives.

Find the full experiment details here: Air Pressure Experiment – Bernoulli Principle

16. Floating Plate Experiment using Atmospheric Pressure

This floating plate experiment is specially designed for parents and teachers to explain atmospheric pressure to the kids in a clear way. This experiment provides you with crystal clear explanations of the basics along with some fun activities.

Let us try this experiment without any hassles: Click here, Floating Plate Experiment using Atmospheric Pressure

17. Smaller Balloon Stronger Balloon Experiment

With this experiment, we are going to explore science and maths together in a brilliant way using simple ingredients available at home. Ask your children to connect two different-sized balloons and predict which way the air flows and why! Analyze their conclusions and teach them the appropriate science behind the experiment.

If you also find it interesting, then click here to know more details on how to perform the experiment: Smaller Balloon Stronger Balloon Experiment

18. Air Pressure Experiment using Straws and Tennis Ball

This is a fun and classic experiment to demonstrate air pressure to the children in an easy way!! Ball in the air keeps children engaged and entertained while learning Air Pressure Science.

Have a look at the experiment here: Air Pressure Experiment using Straws and Tennis Ball

19. Coin Poppers Science Experiment

Easy science experiment to demonstrate air pressure using coins! For young kids, this experiment is like a play while experimenting with coins. But can you use any type of coin!? How do coins demonstrate air pressure? Get the answers to all your questions from the experiment disclosed in detail here: Coin Poppers Science Experiment

20. Exploring Air and Air Pressure Science Experiment

A remarkable experiment to investigate the relation between air and air pressure. Best demonstration experiment for teachers to show children on after school classes about air pressure. Kids will get to know about the air and its properties in a simple way!!

Get the complete details here: Exploring Air and Air Pressure Science Experiment

Hope you have got a handful of the best and classic science experiments that clearly demonstrates Air Pressure. All the experiments are safe, easy-to-perform, easy-to-clean, and learning activities with simple steps and materials available in the home.

Kids also will get to analyze the air properties and how it works on different objects around us in real life. Grab it and experiment hassle-free! Happy Experiments!!

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Force and pressure Worksheet-10

• Define the term atmospheric pressure.
• How would 'thrust' on the bottom of a liquid level change if 'area' is doubled keeping the 'pressure' same?
• Two objects of masses M and 2M are lying on an equal area. Determine the ratio of pressure exerted by them on the ground.
• When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in. the dropper. The rise of water in the dropper is due to

(a) Pressure of water.

(b) Gravity of the earth.

(c) Shape of rubber bulb.

(d) Atmospheric pressure.

• How many objects should be present for a force to come into play?
• Two friends A and B are applying a force of 2 Newton and 4 Newton on a box in the same direction. What will be the total force applied by them?
• In a tug of war, side A applies 10 Newton force and side B applies 8 Newton force. Which side will the rope move?
• What happens to the speed of a body when a force is applied?
• Can we change the direction of the moving object by applying a force?
• What is meant by change in state of motion of the object?
• Is it possible that a force changes the direction of motion but not the speed of an object?
• Give an example to show that force can change the shape of an object.
• What is meant by muscular force?
• A ball is rolled on the ground and it comes to rest after some time even though no force is applied. Why?
• Does the force of friction also act on the objects moving in the air?
• Is it essential for the agent applying a force on an object to be in contact?
• Give one example of a force which can act from a distance.
• What is meant by electrostatic force?
• Is the electrostatic force a contact force or non-contact force?
• What is meant by force of gravitation?
• The pressure exerted by air is known as atmospheric pressure.
• Thrust will also be doubled.

∴ ratio of pressures is 1 : 2

• Atmospheric pressure.
• There should be at least two objects.
• The total force will be 6 Newton, i.e ., the sum of their individual forces.
• The rope will move towards side A as more force is applied by side A.
• The speed of a body can be increased or decreased by applying force.
• Any change in the speed or direction of motion or both, means a change in state of motion of the object.
• Yes, it is possible when a body is moving on a circular path.
• Pressing a rubber ball with the hand.
• The force resulting due to the action of muscles is known as the muscular force.
• The ball comes to rest due to the force of friction acting in the opposite direction to the motion of the ball.
• Yes, air also offers friction to objects moving in air.
• No, the force can also act from a distance. It is known as non-contact force.
• Magnetic force, i.e ., the force exerted by a magnet on another magnet or a piece of iron.
• The force exerted by a charged body is known as electrostatic force.
• It is a non-contact force.
• The force of attraction exerted by the earth on all objects is called the force of gravitation.

• Air Exerts Pressure

Don’t we all just love bubble wrap? Isn’t fun to pop those bubbles? But how is it possible? What is actually happening when you’re popping a bubblewrap? Well, it’s simple! When you pop bubble wrap, the air inside the bubble exerts pressure and hence it pops. Interesting isn’t it? Let us study more in-depth about how air exerts pressure.

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The envelope of air surrounding the earth is called atmosphere . Air has weight. The weight of air presses our bodies all the time . This weight of air acting on a surface causes air pressure . Air pressure can be understood with the help of certain activities.

Browse more Topics under Winds Storms And Cyclones

• Air Expands on Heating
• Thunderstorms and Cyclones
• Materials Required: tumbler, water, square cardboard piece
• Method: Fill up the tumbler with water up to the brim. Cover it with cardboard piece and turn the glass upside down. Slowly remove your hand.
• Observation: Cardboard does not fall and water stays in the glass.
• Inference: air pushes the cardboard up and prevents it from falling.

Activity II

• Materials Required: plastic bottle with cap, hot water, cold water
• Method: Pour hot water into the plastic bottle. Empty the bottle and put the cap tightly. Pour some ice-cold water on it.

• Observation: The bottle will get de-shaped.
• Inference : The air from the bottle expands as it becomes hot. When it is cooled, air contracts. The outside air has more pressure and it crushes the bottle.

Some daily life experiences that show that air exerts pressure

• You find it easier to row the boat when the wind is blowing behind you.
• The wind coming from the back help in flying kite.
• When we suck from the straw, the liquid rises in it.
• The medicine enters the syringe when a piston is pushed out.

High-speed winds are accompanied by reduced air pressure. Let us now perform certain experiments that will show that high-speed winds reduce the air pressure.

Activity III

• Materials Required: Two balloons
• Method: Blow the balloons and tie string to it. Hang them 10 – 12 cm apart on a rim. Blow air in between the balloons.

• Observation: The balloons will move closer.
• Inference : The air pressure between the balloons is reduced due to more speed. Air moves from higher pressure to lower pressure bringing the balloons closer.

Activity IV

• Materials Required: Bottle, crumpled piece of paper
• Method: Crumple a small piece of paper into a ball of a size smaller than the mouth of an empty bottle. Hold the empty bottle on its side and place the paper ball just inside its mouth. Now try to blow on the ball to force it into the bottle.

• Observation: The paper does not move inside.
• Inference : The air pressure at the mouth of the bottle is reduced due to more speed. This forces the air out from the bottle. This prevents the crumpled paper ball to move inside.

Question For You

Q1. Give Reason – Why is the roof of houses blown off when a strong wind is blowing?

Ans: The strong wind above the roof lowers down the air pressure just above the roof. This forces the air from the air to move outward blowing off the roof of the house.

Q2. A child blows air with a straw near the opening of another straw which has its other end in a soft drink bottle. What do you think will happen and why?

Ans: The level of soft drink will rise in the bottle. This will happen because the air pressure is reduced above the straw hence the drink will rise in it.  

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FREE K-12 standards-aligned STEM

curriculum for educators everywhere!

Find more at TeachEngineering.org .

• TeachEngineering
• Air Pressure Experiments: I Can't Take the Pressure!

Hands-on Activity Air Pressure Experiments: I Can't Take the Pressure!

Grade Level: 5 (4-6)

Time Required: 1 hour

Expendable Cost/Group: US $1.00

Group Size: 4

Activity Dependency: None

Associated Informal Learning Activity: I Can't Take the Pressure!

Subject Areas: Algebra

NGSS Performance Expectations:

Curriculum in this Unit Units serve as guides to a particular content or subject area. Nested under units are lessons (in purple) and hands-on activities (in blue). Note that not all lessons and activities will exist under a unit, and instead may exist as "standalone" curriculum.

• Air Composition Pie Charts: A Recipe for Air
• Air - Is It Really There?
• Environmental History Timeline
• Barometric Pressure: Good News – We're on the Rise!
• Dripping Wet or Dry as a Bone?
• Turning the Air Upside Down
• Word Origins & Metaphors: Take Their Word for It!
• Weather Forecasting: How Predictable!

TE Newsletter

Engineering connection, learning objectives, materials list, worksheets and attachments, more curriculum like this, introduction/motivation, troubleshooting tips, activity extensions, activity scaling, user comments & tips.

Air pressure is a concept that is important for engineers from all fields to understand. For instance, environmental engineers must understand air pressure because it affects the way in which air pollution travels through the air. Especially in highly populated areas, engineers work with local communities to understand their unique weather and atmospheric conditions, and suggest public and industry behavior and policy changes to keep the air quality at a safe level for breathing. They also create new prevention technologies that address air pollution at the sources.

After this activity, students should be able to:

• Compare atmospheric pressure (in psi) to the pressure exerted by an object (weight per unit area, in psi).
• Explain why air pressure changes with altitude.
• Identify the locations of high and low pressure in an experiment.
• Describe how engineers must understand air pressure because it affects the way in which air pollution travels via air.
• Identify aspects of pressure that are important to consider in engineering aircraft designs.

Educational Standards Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards. All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN) , a project of D2L (www.achievementstandards.org). In the ASN, standards are hierarchically structured: first by source; e.g. , by state; within source by type; e.g. , science or mathematics; within type by subtype, then by grade, etc .

Ngss: next generation science standards - science.

Common Core State Standards - Math

View aligned curriculum

Do you agree with this alignment? Thanks for your feedback!

International Technology and Engineering Educators Association - Technology

State standards, colorado - math, colorado - science.

Student Activity 1: The Strength of Air Pressure

• activity worksheets (3) and reference sheet, 1 set per student; How Great Is Atmospheric Pressure? - Worksheet 1 , Amount of Air Pressure on a Square Table and Graph - Worksheet 2 , Air Pressure Chart - Worksheet 3 , Air Pressure vs. Altitude Data and Graph Reference Sheet
• graph paper, 1-square-inch grid; 1 sheet per student; online source of printable graph paper: http://www.teachervision.com/lesson-plans/lesson-6169.html
• index cards, 1 per student
• sets of 4 objects that will be weighed, such as a textbook, novel, magazine and dictionary; 1 set per group (have one group weigh themselves as the objects)
• tape, to share with the class
• balance (triple beam, small digital, bathroom scale, etc.), to share with the class

Student Activity 2: Air Pressure and Altitude

• Necco or Vanilla Wafers, or colored tiles/blocks, 14 per student
• paper, pencil, ruler; for each student
• (optional) 1 gallon of water, to show students what 8.5 lbs. of weight feels like

Demo 1: Aluminum Can Crush

• 1 aluminum soda can
• 1 large beaker or bucket
• 1 hot plate
• 1 pair of tongs
• 1 cup tap water
• bucket of ice water
• (optional) trivet, to prevent damage to counter top from heated can

Pressure is defined as the amount of force applied per unit area or as the ratio of force to area (P = F/A). The pressure an object exerts can be calculated if its weight (the force of gravity on an object) and the contact surface area are known. For a given force (or weight), the pressure it applies increases as the contact area decreases.

To better understand this, have students hold a large book flat on their outstretched hands and notice how much pressure the book puts on it. Then, have them try to balance the book on the tip of their index fingers. How much pressure does it seem to exert now?

It is also important to note that air pressure decreases with increasing altitude (see Figure 1 and Table 1). Table 1 lists the air pressure for specific elevations. See the Air Pressure vs. Altitude Data and Graph Reference Sheet for more detailed comparison.

Pressure is measured in various units. Scientists and engineers typically use the metric unit Pascal (Pa). A Pascal is defined as the pressure exerted by a 1 Newton weight (1 kg under Earth's force of gravity) resting on an area of 1 square meter. Below is a list of some of the common units used to measure pressure , and their equivalents. Please note that there are many other units that may be used.

At sea level, the atmospheric air pressure can be represented as any of the following:

• 1.013 x 10 5 Pa (Pascal or N/m 2 )
• 1 atm (atmosphere)
• 760 mm Hg (millimeters of mercury)
• 14.7 lb/in 2 (psi, pounds force per square inch; if 1-pound weight rests on 1-square inch of surface area, the pressure is 1 psi)

Humans are relatively permeable to air (it can move easily in and out of our bodies) and that is why our internal pressure stays the same as the pressure of the surrounding (ambient) air. This is the same reason why fish are not crushed in the depths of the ocean; they are permeable to water. Although the atmosphere exerts a significant amount of pressure on everything in our environment, the only time most people are aware of air pressure is when it changes (such as changes in altitude, for example, as you drive up a mountain).

As you climb in elevation, the atmospheric pressure decreases while the pressure in your middle ear may remain constant, causing a difference in pressure. This pressure difference causes your eardrums to bulge and possibly produce pain. Yawning relieves the pain because the action opens the small Eustachian tubes between your ear and pharynx allowing air to escape from your middle ear into the atmosphere though your nose and mouth. As the pressure is equalized, your ear "pops" when the eardrum snaps back into its normal position.

Engineers who design airplanes study air pressure. Airplane cabins are "pressurized." This means the inside of the plane maintains a constant pressure of about 14 pounds per square inch regardless of the pressure outside of the cabin. At high altitudes, the air has a very low pressure, which affects the way we breathe. This same effect occurs when people move from sea level locations, such as New York City, to the mountains, such as Denver, CO. Often, it takes a few weeks for their bodies to adjust to the lower pressure.

Before the Activity

• Gather materials and make copies of the reference sheet and three worksheets ( 1 , 2 , 3 ).
• If balances and scales are not available in your classroom, determine the mass of the objects before class and provide students with the information.
• Practice the aluminum can demonstration.
• Ask students to define air pressure. If necessary, remind them the properties of air: it has mass, it takes up space, it can move, it exerts pressure (it pushes on things) and it can do work.
• Ask: How strong is atmospheric air pressure? (Is it as much pressure as an ant standing on 1 square inch would exert? Or, an elephant? Or, 12 elephants?)
• Tell students they are going to compare the pressure that different objects exert on the Earth (due to gravity) to atmospheric air pressure.
• Divide the class into groups of four students each.
• Distribute to each group the worksheets, graph paper, index cards and four objects (for one group, the four objects could be themselves).
• Have the students determine the mass of their objects and record it on the worksheet 1 (see Figure 2). Direct the group that is weighing themselves to each stand on one flat foot on the scale while the measurement is made.

• Direct students to place their object on the grid paper in the same orientation as it was when it was on the balance (the position does not affect the mass, but it affects the contact/surface area value and thus, the ultimate pressure). Have students carefully trace around the object, add up the squares and record the contact area on their worksheets. Have the group that is weighing themselves trace around the foot they stood on. Students may need some help estimating and rounding for partial squares.
• Have students record on their worksheets the data for every group member.
• Ask students to calculate the pressure that each of the objects exerts. (P = F/A, in this case F = weight of the object.)
• Have students write the name of their objects and the resulting pressures on index cards and tape them to the classroom board.
• Have students rearrange the cards in order of increasing pressure.
• On their worksheets, have students predict which object they think has the closest value to the air pressure around them and explain why. Ask a few students to share their predictions.
• Share the actual value of the air pressure with the students (about 14.7 psi at sea level). Were they surprised with the results?
• Ask the class: Does air pressure change with altitude? If so, how does it change? Why do they think this happens?
• Direct students to each build a tower using wafers or colored tiles/blocks that is 14-wafers tall (see Figure 3).

• Ask students: How does this model represent air pressure changing with altitude? (Listen to student explanations.) Explanation: Imagine that the wafers are the air in the atmosphere and that the bottom wafer is at sea level—the lowest point in the troposphere. The top wafer is a higher layer in the stratosphere or some place like the top of Mount Kilimanjaro. Imagine that you are standing at sea level, the level of the bottom wafer. The air pressure at sea level is the highest, because at that point all the air (wafers) is pressing on everything. Now imagine that you are standing on/near the top of the stack, at a higher altitude. Here, much less air (fewer wafers) are pressing on each other, thus the air pressure is less than at sea level.
• Share the sea level air pressure with students (14.7 psi) and the air pressure in your city (for example, Denver, CO, at one-mile high, is about 12.4 psi).
• Ask students to describe in their own words how air pressure changes with altitude, recording their information on worksheet 1.
• Variation: Stack books or pillows in students' laps/arms so they can "feel" the different pressures instead of just visualizing with the wafers.
• Eat the candy or cookie wafers.
• In Denver, the Earth's atmosphere has a force of about 12 pounds per square inch (psi). For reference, a gallon of milk or water weighs about 8 pounds. Show the students what a 1 inch by 1 inch square looks like. Now show the students what a 2 x 2-inch square looks like, and ask them how many pounds would be pressing down on that square. (Answer: 48) See the Amount of Air Pressure on a Square - Worksheet 2 , for a comparison of pressures at the altitudes of Boston, MA, and Denver, CO.
• Ask: How many pounds would be pressing on a 3 x 3-inch square? (Answer: 108) A 4 x 4-inch? (Answer: 192) Direct the students to complete the  Air Pressure Chart - Worksheet 3 .
• Ask: Do you see a pattern? What happens every time the square increases by one in 2 ? (Answer: The pounds of force increases by 12.)
• The average pressure on a middle school student is 24,000 pounds! Ask: Do you feel that pressure? Why don't you feel that pressure? (See if students can explain. Answer: Humans are permeable to air. Air exists inside the body, too—from breathing, through the skin, ears, etc.—and that air balances out the pressure on the outside of the body.)
• Fill the bucket with ice water.
• Fill the soda can with approximately 1 cm of water.
• Place the soda can on the hot plate until the water boils. Be alert to not let the can boil dry!
• Use the tongs to carefully remove the can from the heat and place it in an upright position on the tabletop (or trivet).
• Ask: Do you see any change in the can? (See Figure 4.) Direct students to record their observations on worksheet 1
• Repeat the heating process. This time, when you remove the can with the tongs, quickly invert it and submerge the can opening in the bucket of ice water.
• Ask: Do you see any change in the can? (See Figure 4.) Direct students to record their observations on worksheet 1.

• Direct students to draw a diagram of the experimental results. Have them indicate where the pressure must be the highest with a letter H and the lowest with a letter L. (Answer: Air pressure is lowest, L, inside the overturned can and highest, H, outside the can and around the experiment.)
• Ask: Why do you think the can was crushed? (Listen to some student explanations. Answer: Before heating, the pressure inside and outside the can is the same. We assume the pressures on both sides remain approximately the same while heating since the can does not deform. As the water boils, the air that escapes from the can is gradually replaced by water vapor until the internal atmosphere is composed almost completely of water vapor. When the can is removed from the heat, the vapor pressure drops dramatically. It decreases from 101.3 kPa at 100 ºC to about 2.3 kPa at room temperature. Therefore, as the temperature drops to room temperature, the pressure inside the can drops 97%. If the can is open to the atmosphere, air flows back into the can as the water condenses and keeps the pressure essentially constant. However, if the opening of the can is submerged, the vapor in the can cannot equilibrate with the atmosphere. In the bucket of water, the temperature in the can decreases and the water vapor condenses, creating a pressure difference of almost 99 kPa. Water is forced in to fill this partial vacuum, but before it does, air pressure on the walls implodes the can. Note that the collapsed can contains water (more than when you started), indicating water entered at the same time the walls collapsed.
• Have students work in pairs to answer the following questions:
• The air inside an aircraft is kept at a pressure similar to what human bodies experience at the Earth's surface. Knowing this, what can you say about the pressure difference between the air inside a plane versus the air outside a plane, once a plane is 30,000 ft above the Earth's surface? (Answer: The air pressure is much lower outside the plane than inside the plane.)
• Is pressure pushing from the inside of the plane outwards? Or, is pressure pushing on the outside on the plane inwards? It may help to figure this out by sketching a plane and using arrows to indicate the direction of pressure. (Answer: Pressure is pushing from the inside [high pressure] to the outside where the pressure is lower.)
• How might engineers incorporate this knowledge into their airplane designs? (Answer: Engineers design airplanes, jets, rockets and space shuttles to be strong enough so they do not explode when high in the atmosphere and in conditions in which the inside and outside air pressures are different. The material needs to be much stronger than an aluminum can!)

Pre-Activity Assessment

Discussion Questions : Solicit, summarize and integrate student responses to the following questions. After the discussion, explain that these questions will be answered during the upcoming demonstrations and activities. Ask the students:

• What is air pressure?
• How strong is atmospheric air pressure? Is it as much pressure as an ant standing on 1 square inch would exert? Or, an elephant? Or, 12 elephants?

Activity Embedded Assessment

Activity Sheets : Use the three worksheets and reference sheet to help students follow along with the activity. Review their answers to gauge their depth of comprehension.

Post-Activity Assessment

Student-Generated Questions : Ask each student to come up with one question to ask the class, based on the content of the activity. The students may require help in generating the questions. Call on a few students to ask their questions.

Safety Issues

• Make sure that students understand that they could get burned if they touch the hot plate or hot can.
• Make sure the hot plate is turned off when not in use.

In English, we use the term "weight" when we really mean mass. Mass is the amount of matter in an object. Weight is the force of gravity on a particular mass. Students may need some clarification. To add to the confusion, we also use the unit of pounds for both! However, mass is measured in pounds-mass and weight in pounds-force.

During the calculation of contact area, students may need some help estimating and rounding for partial squares. It may help to do a quick example on the classroom board or overhead projector.

You may want to start the water boiling in the aluminum can while conducting Student Activity 2: Air Pressure and Altitude—just do not forget about it and let it boil dry!

When the can is dunked in the bucket of cold water, it is crushed very quickly, so have students gather around so they can see what happens. It is highly recommended that you practice this activity in advance.

If calculating pressures exerted at sea level is too difficult, it may be easier to provide the square areas 1-12 or perform the calculations using the air pressure in Denver (12 psi).

Have students do all their measurements and calculations in metric units. Use the following conversion factors:

1 cm 2 = 0.001 m 2

1 lb = 0.454 kg

1 in 2 = 6.45 cm 2 = 0.000645 m 2

1 Pa = 1.45 x 10 -4 lb/in 2

1 kg mass weighs 9.8 N

Change the size of the grid students use to calculate the surface area of their feet. For example, use a 1 cm 2 grid, or a ½ in 2 grid.

Make a graph that shows how air pressure changes with altitude.

Relate the concepts explored in this activity to water pressure deep in the ocean.

• For grades 3 and 4, the multiplication and division may need to be modified; expect students to be able to do the multiplication with a calculator.
• For grades 1 and 2, conduct this activity as a class. Use tape and an index card to label items with the pressure that they exert, and have each student take a card. Ask students to arrange themselves (and the cards) in order of increasing pressure.

For grade 6 students:

• Rather than demonstrate the squares to the students, have them measure their own 1 x 1, 2 x 2, 3 x 3, and 4 x 4-inch square and find the pressure.
• The average surface area for an elementary school student is about 2,000 in 2 . Rather than telling students this information, have them calculate the amount of air pressure pushing down on them (24,000 lbs.).
• Have students calculate the force for other areas such as one square foot (144 in 2 ), a football field (approx. 8,000,000 in 2 ).
• Have students plot square inches vs. force on a graph.
• The average force of the atmosphere at sea level (New York City = 87 ft., San Diego = 13 ft. and Boston = 10 ft. — all close to sea level) is 15 pounds per square inch (almost 2 gallons of milk). Have students repeat their calculations for the pressure a sea level.

For grade 3 students:

• The average force of the atmosphere at sea level (New York City = 87 ft., San Diego = 13 ft. and Boston = 10 ft.—all close to sea level) is 15 pounds per square inch (almost 2 gallons of milk). Have students repeat their calculations for the pressure at sea level.
• Have students complete the Amount of Air Pressure on a Square - Worksheet 2 , and make predictions for several other squares such as 100 x 100.

For grade 2 students, simplify the psi (pounds per square inch) from 12 to 10 for easier calculations.

Students build and observe a simple aneroid barometer to learn about changes in barometric pressure and weather forecasting.

Air pressure is pushing on us all the time although we do not usually notice it. In this activity, students learn about the units of pressure and get a sense of just how much air pressure is pushing on them.

Students learn about the fundamental concepts important to fluid power, which includes both pneumatic (gas) and hydraulic (liquid) systems.

Students learn about the underlying engineering principals in the inner workings of a simple household object – the faucet. Students use the basic concepts of simple machines, force and fluid flow to describe the path of water through a simple faucet.

Cunningham, J. and Herr, N. Hands-on Physics Activities with Real-Life Application . West Nyack, NY: The Center for Applied Research in Education, p. 188-210, 1994.

Quarter-Inch Graph Paper (printable). Copyright 2000-2004. Teacher Vision, Family Education Network, Pearson Education, Inc. (source of printable graph paper) Accessed on September 17, 2020. http://www.teachervision.com/lesson-plans/lesson-6169.html

Walpole, Brenda. 175 Science Experiments to Amuse and Amaze Your Friends . Random House, p. 72, 1988.

UNESCO. 700 Science Experiments for Everyone . New York, NY: Doubleday, p. 79, 1958.

Contributors

Supporting program, acknowledgements.

The contents of this digital library curriculum were developed under grants from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and the National Science Foundation (GK-12 grant no. 0338326). However, these contents do not necessarily represent the policies of the Department of Education or the National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: September 17, 2020

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Top Ten Air Pressure Experiments to Mystify Your Kids Using Stuff From Around the House

What is air pressure, magic water glass trick, plumber magic, magic egg trick, fountain bottle, ping pong funnel, squished soda can, squished balloon, the million dollar bet, flying papers, kissing balloons.

Disclaimer: Experiments should only be attempted with adult supervision and are done at your own risk. 

There's air surrounding us everywhere, all at the same pressure of 14.7 psi (pounds per square inch). It's the same force you feel on your skin whether you're on the ceiling or the floor, under the bed, or in the shower.

An interesting thing happens when you change a pocket of air pressure - things start to move. This difference in pressure that causes movement is what creates winds, tornadoes, airplanes to fly, and some of the experiments we're about to do right now.

An important thing to remember is that higher pressure always pushes things around. (Meaning lower pressure does not "pull", but rather that we think of higher pressure as a "push".)

Another interesting phenomenon occurs with fast-moving air particles. When air moves fast, it doesn't have time to push on a nearby surface, like an airplane wing. It just zooms by, barely having time to touch the surface. The air particles are really in a rush.

Think of really busy people driving fast in their cars. They are so busy doing other things and driving fast to get somewhere that they don't have time to just sit and relax.

Air pressure works the same way. When the air zooms by a surface (like an airplane wing) like fast cars, the fast air has no time to push on the surface and just sit there, so not as much air weight gets put on the surface.

Less weight means less force on the area. (Think of "pressure" as force on a given area or surface.) This causes a less (or lower) pressure region wherever there is faster air movement.

Confused? Great! Let's try some experiments out to straighten out these concepts so they make sense to you.

Fill a glass one-third with water. Cover the mouth with an index card and invert (holding the card in place) over a sink. Remove your hand from the card. Voila!

The card stays in place because air is heavier than water, and the card experiences about 15 pounds of force pushing upward by the air and only about one pound of force pushing downward from the water - hence the card stays in place. (Try this trick over someone's head when you get good at it.)

Take two clean old-fashioned, red rubber-and-wood-stick plungers and stick them together (you may need to wet the rims first). Try to separate them.

Why is it so hard? When you rammed them together, air was forced out of the cavity that the insides make when pushed together, leaving you with a lower air pressure pocket inside, compared to the surrounding air pressure of 14.7 pounds per square inch (psi) outside the plungers. Higher pressure always pushes and thus is keeping the plungers together.

Remove the shell from a hard-boiled egg and use a bottle with a neck large enough that the egg can be squeezed through (without squashing it) – old fashioned milk bottles work great. Light a match and toss it in, quickly setting the egg (small-end down) on the mouth of the bottle.

The air inside gets used up by the flame, lowering the air pressure inside the bottle. The higher pressure, now outside the bottle, pushes on the egg and pops it in. (To remove the egg, turn the bottle upside down and get the egg to be small-end down inside the bottle near the mouth. Blow hard into the mouth of the bottle.)

Seal a 2-liter soda water bottle (half-full of water) with a lump of clay wrapped around a long straw, sealing the straw to the mouth of the bottle. Blow hard into the straw.

As you blow air into the bottle, the air pressure increases. This higher pressure pushes on the water, which gets forced up and out the straw.

Insert a ping pong ball into a funnel and blow hard. (You can tilt your head back so that the ball end points to the ceiling. Can you blow hard enough so when you invert the funnel, the ball stays inside? Can you pick up a ball from the table?

As you blow into the funnel, the air where the ball sits in the funnel moves faster and generates lower air pressure than the rest of the air surrounding the ball. This means that the pressure under the ball is lower than the surrounding air which is, by comparison, a higher pressure. This higher pressure pushes the ball back into the funnel… no matter how hard you blow or which way you hold the funnel.

Heat an empty soda can (large beer cans actually will work better if you have one) in a skillet with a few tablespoons of water in the can over a hot stove. Have a shallow dish with about ¼ inch of ice water handy (enough water to make a seal with the top of the can). When the can emits steam, grab the can with tongs and quickly invert it into the dish. CRACK!

The air in the can was heated, and things that are hot tend to expand. When you cool it quickly by taking it off the stove onto a cold plate, the air cools down and shrinks, creating a lower pressure inside. Since the surrounding air outside of the can is now higher, it pushes on all sides of the can and crushes it.

Blow a balloon up so that it is just a bit larger than the opening of a large jam jar and can't be easily shoved in. Light a small wad of paper towel on fire and drop it into the jar. Place the balloon on top. When the fire goes out, lift the balloon… and the jar goes with it!

The air gets used up by the flame and lower the air pressure inside the jar. The surrounding air outside, now at a higher pressure than inside the jar, pushes the balloon into the jam jar.

Take an empty water or soda bottle and lay it down horizontally on a table. Carefully set a small wadded up ball of paper towel in the mouth of the bottle. (The ball should be about half the size of the opening.) I bet you a million dollars that you can't blow hard and get the paper to go into the bottle!

Why is this so impossible? You're trying to force more air into the bottle, but there's no room for the air already inside to go except back out the mouth of the bottle, taking the paper ball with it.

Hold a regular sheet of paper to your bottom lip (you may have to play a bit to find the exact location) and blow hard across the sheet. The sheet flies up!

This is the same reason airplanes can fly. As you blow across the top of the sheet, you lower the air pressure (because the air is moving faster), and thus the pressure on the underside of the sheet is now higher, and higher air pressure pushes the sheet upwards.

Blow up two balloons. Attach a piece of sting to each balloon. Have each hand hold one string so that the balloons are at nose level, 6" apart. Blow hard between the balloons and watch them move!

The air pressure is lowered as you blow between the balloons (think of the air molecules as ping pong balls … they balls don't have enough time to touch the balloon surface as they zoom by). The air surrounding the balls that's not really moving is now at a higher pressure, and pushes the balloons together.

If you enjoyed this experiment and want more, head over to Aurora's website and get your free copy of her Homeschool Science Guide . _____________________________________

Since 1996, Aurora Lipper has been helping families learn science. As a pilot, astronomer, mechanical engineer and university instructor, Aurora can transform toilet paper tubes into real working radios and make laser light shows from Tupperware.

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