case study questions class 9 physics work and energy

Case Study Questions Class 9 Science Work and Energy

Case study questions class 9 science chapter 11 work and energy.

CBSE Class 9 Case Study Questions Science Work and Energy. Important Case Study Questions for Class 9 Exam. Here we have arranged some Important Case Base Questions for students who are searching for Paragraph Based Questions Work and Energy.

CBSE Case Study Questions Class 9 Science – Work and Energy

(i) Work done is

(b) Vector quantity

(a) positive

(b) Newton meter(N-m)

(2) A moving object can do work. An object moving faster can do more work than an identical object moving relatively slow. A moving bullet, blowing wind, a rotating wheel, a speeding stone can do work. How does a bullet pierce the target? How does the wind move the blades of a windmill? Objects in motion possess energy. We call this energy kinetic energy.

The energy possessed by an object is thus measured in terms of its capacity of doing work. The unit of energy is, therefore, the same as that of work, that is, joule (J).

(b) Kinetic energy and work

(c) Both a and b

to that point against gravity.Let the work done on the object against gravity beW. That is,

(c) Nuclear energy

= 5 ×9.8 ×10

(a) Electric energy converted into heat energy

(c) Potential energy

(ii) Total energy consumed divided by total time taken is called as

Time used, t = 5 h

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Class 9 Science Case Study Questions Chapter 11 Work and Energy

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Case study Questions in Class 9 Science Chapter 11  are very important to solve for your exam. Class 9 Science Chapter 11 Class 9 Science Case Study Questions have been prepared for the latest exam pattern. You can check your knowledge by solving case study-based questions for Class 9 Science Chapter 11 Work and Energy

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In CBSE Class 9 Science Paper, Students will have to answer some questions based on Assertion and Reason. There will be a few questions based on case studies and passage-based as well. In that, a paragraph will be given, and then the MCQ questions based on it will be asked.

Work and Energy Case Study Questions With Answers

Here, we have provided case-based/passage-based questions for Class 9 Science  Chapter 11 Work and Energy

Case Study/Passage-Based Questions

Case Study 1: The figure shows a watch glass embedded in clay. A tiny spherical ball is placed at edge B at a height h above the center A

The kinetic energy of the ball, when it reaches point A is (a) zero (b) maximum (c) minimum (d) can’t say

Answer: (b) maximum

The ball comes to rest because of (a) frictional force (b) gravitational force (c) both (a) and (b) (d) none of these

Answer: (c) both (a) and (b)

The energy possessed by the ball at point C is (a) potential energy (b) kinetic energy (c) both potential and kinetic energy (d) heat energy.

Answer: (a) potential energy

Case Study 2: The principle of conservation of energy states that the energy in a system can neither be created nor be destroyed. It can only be transformed from one form to another, but the total energy of the system remains constant. Conservation of electrical energy to various forms or vice versa along with devices is illustrated in the figure given below.

Water stored in a dam possesses (a) no energy (b) electrical energy (c) kinetic energy (d) potential energy.

Answer: (d) potential energy.

A battery lights a bulb. Describe the energy changes involved in the process. (a) Chemical energy →Light energy → Electrical energy (b) Electrical energy → Chemical energy → Electrical energy (c) Chemical energy → Electrical energy → Light energy (d) None of these.

Answer: (c) Chemical energy → Electrical energy → Light energy

Name a machine that transforms muscular energy into useful mechanical work. (a) A microphone (b) Bicycle (c) Electric torch (d) An electric bell

Answer: (b) Bicycle

A body is falling from a height of h. After it has fallen a height h/2 , it will possess (a) only potential energy (b) only kinetic energy (c) half potential and half kinetic energy (d) more kinetic and less potential energy.

Answer: (c) half potential and half kinetic energy

Case Study 3: An elevator weighing 500 kg is to be lifted up at a constant velocity of 0.4 m s –1 . For this purpose, a motor with the required horsepower is used

The power of the motor is (a) 1940 W (b) 1950 W (c) 1960 W (d) 1970 W

Answer: (c) 1960 W

The power of motor in hp is (a) 2.33 (b) 2.43 (c) 2.53 (d) 2.63

Answer: (d) 2.63

Case Study 4: Work and energy are fundamental concepts in physics that help us understand the physical world and the processes happening around us. Work is done when a force is applied to an object, and the object moves in the direction of the applied force. It is calculated as the product of force and displacement. The unit of work is joule (J). Energy, on the other hand, is the ability to do work. It exists in different forms, such as kinetic energy, potential energy, and various other forms like thermal energy, electrical energy, and chemical energy. The law of conservation of energy states that energy cannot be created or destroyed, but it can be transformed from one form to another. Understanding the concepts of work and energy helps us analyze the efficiency of machines, calculate the amount of work done, and comprehend various physical phenomena.

When is work considered to be done on an object? a) When a force is applied to the object b) When the object moves in the direction of the applied force c) When the object remains stationary d) When the object changes its shape Answer: b) When the object moves in the direction of the applied force

How is work calculated? a) Force multiplied by velocity b) Force multiplied by acceleration c) Force multiplied by displacement d) Force divided by time Answer: c) Force multiplied by displacement

What is the unit of work? a) Newton (N) b) Meter (m) c) Joule (J) d) Watt (W) Answer: c) Joule (J)

What is energy? a) The ability to do work b) The force applied to an object c) The distance traveled by an object d) The mass of an object Answer: a) The ability to do work

According to the law of conservation of energy, what happens to energy? a) It can be created b) It can be destroyed c) It can be transformed from one form to another d) It remains constant Answer: c) It can be transformed from one form to another

Hope the information shed above regarding Case Study and Passage Based Questions for Class 9 Science Chapter 11 Work and Energy with Answers Pdf free download has been useful to an extent. If you have any other queries about CBSE Class 9 Science Work and Energy Case Study and Passage Based Questions with Answers, feel free to comment below so that we can revert back to us at the earliest possible By Team Study Rate

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Case Study and Passage Based Questions for Class 9 Science Chapter 11 Work and Energy

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Case Study Questions for Class 9 Science Chapter 11 Work and Energy

In CBSE Class 9 Science Paper, Students will have to answer some questions based on Assertion and Reason. There will be a few questions based on  case studies and passage based  as well. In that, a paragraph will be given, and then questions based on it will be asked.

Here, we have provided case based/passage based questions for Class 9 Science  Chapter 11 Work and Energy . Students can practice these questions for their exam.

Case Study/Passage Based Questions

Question 1:

Read the following and answer any four questions from (i) to (iii).

Figure shows a watch glass embedded in clay. A tiny spherical ball is placed at the edge B at a height h above the centre A.

(i) The kinetic energy of ball, when it reaches at point A is (a) zero (b) maximum (c) minimum (d) can’t say.

(ii) The ball comes to rest because of (a) frictional force (b) gravitational force (c) both (a) and (b) (d) none of these.

(iii) The energy possessed by ball at point C is (a) potential energy (b) kinetic energy (c) both potential and kinetic energy (d) heat energy.

You may also like:

Case study and passage based questions for other chapters of class 9 science is given below.

Chapter 3 Atoms and Molecules

Chapter 4 Structure of Atom

Chapter 10 Gravitation

Chapter 11 Work and Energy

Chapter 13 Why Do We Fall Ill?

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Class 9th Science - Work and Energy Case Study Questions and Answers 2022 - 2023

Class 9th Science - Work and Energy Case Study Questions and Answers 2022 - 2023 Study Materials Sep-09 , 2022

QB365 provides a detailed and simple solution for every Possible Case Study Questions in Class 9th Science Subject - Work and Energy, CBSE. It will help Students to get more practice questions, Students can Practice these question papers in addition to score best marks.

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Work and energy case study questions with answer key.

Final Semester - June 2015

(ii) What is the value of total energy of the bob at position A ?

(iii) What is the value of kinetic energy of the bob at mean position 'O' ?

(iv) What is the value of kinetic energy and potential energy of the bob at the position 'P' whose height above 'O' is 2 cm ?

(v) What is kinetic energy? (a) Energy acquired due to motion (b) Energy acquired due to rest (c) Sum of Potential and mechanical energy (d) It is the energy stored inside a body.

*****************************************

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Work and Energy

Class 9 - ncert science solutions, intext questions 1.

A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?

Force (f) = 7 N

Displacement (S) = 8 m

Work done = Force × Displacement

Substituting we get,

W = 7 × 8 = 56 Nm or 56 J

Hence, work done = 56 J

Intext Questions 2

When do we say that work is done?

Work is said to be done when force applied on an object shows the displacement in that object. It is equal to the product of force and displacement.

Work done = force x displacement

Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Define 1 J of work.

1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.

A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Force (F) = 140 N

Displacement (S) = 15 m

W = 140 x 15 = 2100 J

Hence, 2100 J of work is done in ploughing the length of the field.

Intext Questions 3

What is the kinetic energy of an object?

The kinetic energy is the energy possessed by an object due to its motion. It increases when the speed increases.

Write an expression for the kinetic energy of an object.

Kinetic Energy (K E ) = 1 2 \dfrac{1}{2} 2 1 ​ mv 2 . Its SI unit is Joule (J).

The kinetic energy of an object of mass m, moving with a velocity of 5 ms -1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Kinetic Energy (K E ) = 25 J

Velocity (v) = 5 ms -1

Kinetic Energy (K E ) = 1 2 \dfrac{1}{2} 2 1 ​ mv 2 .

25 = 1 2 \dfrac{1}{2} 2 1 ​ x m x 5 2

25 x 2 = 25 x m

m = 50 25 \dfrac{50}{25} 25 50 ​

When velocity is doubled :

v' = 10 ms -1

K E = 1 2 \dfrac{1}{2} 2 1 ​ x 2 x 10 2

K.E. = 100 J

When velocity is increased three times, then

v'' = 15 ms -1

K E = 1 2 \dfrac{1}{2} 2 1 ​ x 2 x 15 2

K.E. = 225 J

Hence, Kinetic Energy becomes 100 J when velocity is doubled and it becomes 225 J when velocity is increased three times.

Intext Questions 5

What is power?

Power is defined as the rate of doing work or the rate of transfer of energy. If an agent does a work W in time t, then power is given by:

Power = Work done Time \dfrac{\text{Work done}}{\text{Time}} Time Work done ​

It is expressed in watt (W).

Define 1 watt of power.

1 watt is the power of an agent, which does work at the rate of 1 joule per second.

A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Time = 10 s

Work done = Energy consumed by the lamp = 1000 J

Power = 1000 10 \dfrac{1000}{10} 10 1000 ​ = 100 Js -1 or 100 W

Hence, the power of the lamp is 100 W

Define average power.

Average power is defined as the ratio of total energy consumed to the total time taken by the body.

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

(a) Suma is swimming in a pond.

(b) A donkey is carrying a load on its back.

(c) A wind-mill is lifting water from a well.

(d) A green plant is carrying out photosynthesis.

(e) An engine is pulling a train.

(f) Food grains are getting dried in the sun.

(g) A sailboat is moving due to wind energy.

Work is said to be done when force applied on an object shows the displacement in that object.

(a) While swimming, Suma applies a force to push the water backwards. She swims in the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, the work is done .

(b) While carrying a load, the donkey has to apply a force in the upward direction. But, displacement of the load is in the forward direction. Since displacement is perpendicular to force, the work done is zero.

(c) A windmill works against gravity to elevate water. The windmill lift water by applying a force in an upward direction, and thus the water is moving in the same upward direction itself. Hence, work is done .

(d) No force is required when a green plant is carrying out photosynthesis and there is no displacement of plant. Hence, no work is done .

(e) When an engine is pulling a train, it is applying a force in the forward direction and the train is moving. As, displacement and force are in the same direction. Hence, work is done .

(f) As there is no force applied when grains are dried and there is no displacement as well. Hence, no work is done .

(g) When a sailboat is moving due to wind energy, it is applying force in the forward direction. So, it is moving in the forward direction. As, displacement and force are in the same direction. Hence, work is done .

An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Work done by gravity depends on the vertical displacement of the body.

So, work done by gravity is :

Vertical displacement, h = 0 [∵ initial and the final points of the path of the object lie on the same horizontal line.]

∴ W = m × g × 0 = 0

Hence, work done by the force of gravity on the object = 0.

A battery lights a bulb. Describe the energy changes involved in the process.

A battery converts chemical energy into electrical energy. When the bulb receives this electrical energy, it converts it into light and heat energy. Hence, energy changes involved in the process are :

Chemical Energy ⟶ Electrical Energy ⟶ Light Energy + Heat Energy.

Certain force acting on a 20 kg mass changes its velocity from 5 ms -1 to 2 ms -1 . Calculate the work done by the force.

Initial velocity u = 5 ms -1

Mass of the body = 20 kg

Final velocity v = 2 ms -1

Initial kinetic energy

E i = 1 2 \dfrac{1}{2} 2 1 ​ mu 2

E i = 1 2 \dfrac{1}{2} 2 1 ​ x 20 x 5 2 = 10 × 25 = 250 J

Final kinetic energy

E f = 1 2 \dfrac{1}{2} 2 1 ​ mv 2 = 1 2 \dfrac{1}{2} 2 1 ​ x 20 x 2 2 = 10 × 4 = 40 J

As, Work done = Change in kinetic energy = E f – E i

Work done = 40 J - 250 J

Work done = -210 J

Hence, work done by the force = -210 J.

A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.

Mass (m) = 10 kg

Work done by gravity depends on the vertical displacement of the body. It is independent of the horizontal path.

So, work done by gravity is = m g h

Vertical displacement, h = 0 [∵ the line joining A and B is horizontal. ]

Hence, work done on object by gravity is zero.

The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

No. When the body falls from a height, its potential energy changes into kinetic energy progressively. A decrease in the potential energy is equal to an increase in the kinetic energy of the body. Hence, throughout the process, the total mechanical energy of the body remains conserved.

Hence, the law of conservation of energy is not violated.

What are the various energy transformations that occur when you are riding a bicycle?

The rider's muscular energy is converted to heat energy and the bicycle's kinetic energy while riding a bicycle. Hence, energy transformations are :

Muscular energy ⟶ Kinetic energy + Heat energy

Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

When we push a huge rock and fail to move it, our muscular energy is not transferred to the rock as kinetic energy as there is no displacement of the rock. However, as per the law of conservation of energy, our muscular energy is transformed into heat energy that heats up our body and makes us sweat.

A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

Energy (E) = 250 units

1 kWh = 3.6 x 10 6 J

1 unit of energy = 1 kWh

So, 250 units of energy = 250 × 3.6 × 10 6 = 9 × 10 8 J.

Hence, 250 units of energy = 9 × 10 8 J.

Question 10

An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

Mass (m) = 40 kg

Acceleration due to gravity = 10 ms -2

Height (h) = 5 m

Potential energy = ?

Potential energy = m g h

P.E. = 40 × 10 × 5 = 2000 J

Height when halfway down : = 5 2 \dfrac{5}{2} 2 5 ​ = 2.5 m

Potential energy when halfway = ?

P.E. = 40 × 10 × 2.5 = 1000 J

According to the law of conservation of energy:

Total potential energy = potential energy halfway down + kinetic energy halfway down

2000 = 1000 + K.E. halfway down

K.E. at halfway down = 2000 - 1000 = 1000 J

Hence, Potential energy = 2000 J and kinetic energy at halfway down = 1000 joules .

Question 11

What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

When a satellite revolves around the earth in a circular orbit the work done is zero as force of gravity acting on satellite is perpendicular to its displacement.

Question 12

Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher

Yes, a uniformly moving object can experience displacement even when no force is acting on it. According to Newton's first law of motion, an object in motion will continue in its straight-line motion unless acted upon by an external force. Therefore, displacement of an object can occur in the absence of any force acting on it.

Question 13

A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.

Work is said to be done when force applied on an object shows the displacement in that object. In this case, as there is no displacement of the hay bundle, hence no work is done.

Question 14

An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Power of the heater = 1500 W

Converting W to kW

1000 W = 1 kW

So, 1500 W = 1500 1000 \dfrac{1500}{1000} 1000 1500 ​ = 1.5 kW

Time taken = 10 h

Power = Energy consumed Time taken \dfrac{\text{Energy consumed}}{\text{Time taken}} Time taken Energy consumed ​

Energy consumed = Power x Time taken

Energy consumed = 1.5 x 10 = 15 kWh

Converting kWh to J

So, 15 kWh = 3.6 x 10 6 x 15 = 5.4 x 10 7 J

Hence, the energy consumed = 5.4 x 10 7 J

Question 15

Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?

The law of conservation of energy states that energy can only be converted from one form to another; it can neither be created or destroyed. The total energy before and after the transformation remains the same.

Refer the figure of an oscillating pendulum bob shown below:

The kinetic energy decreases and the potential energy becomes maximum at B.

After a moment the the to and fro movement starts again.

So, from B to A, again the potential energy changes into kinetic energy and this process repeats again and again.

So, when the bob is in its state of to and fro movement it has potential energy at the extreme position B or C and kinetic energy at resting position A.

It has both the kinetic energy and potential energy at an intermediate position. However, the sum of kinetic and potential energy remain same at every point of movement.

The bob will eventually come to rest due to the frictional resistance offered by air on the surface of bob and pendulum loses its kinetic energy to overcome this friction and finally comes to rest.

The law of conservation of energy is not violated because the kinetic energy lost by the pendulum to overcome the friction is gained by surroundings. Hence, total energy of the system will remain conserved.

Question 16

An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?

The kinetic energy of an object of mass m, moving with a velocity, v, is given by the expression,

Kinetic energy = 1 2 \dfrac{1}{2} 2 1 ​ mv 2

In order to bring it to rest, its velocity has to be reduced to zero.

An external force has to absorb energy from the object, i.e., do negative work on it, equal to its kinetic energy = − 1 2 -\dfrac{1}{2} − 2 1 ​ mv 2

Question 17

Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 kmh -1 .

Mass (m) = 1500 kg

Initial velocity (v) = 60 kmh -1

Final velocity = 0

Converting kmh -1 to ms -1 : multiply by 5 18 \dfrac{5}{18} 18 5 ​

60 x 5 18 \dfrac{5}{18} 18 5 ​ = 50 3 \dfrac{50}{3} 3 50 ​ ms -1

Work required to stop the moving car = Kinetic energy of car (K.E.)

K.E. = 1 2 \dfrac{1}{2} 2 1 ​ mv 2

= 1 2 \dfrac{1}{2} 2 1 ​ x 1500 x ( 50 3 ) (\dfrac{50}{3}) ( 3 50 ​ ) 2

= 1 2 \dfrac{1}{2} 2 1 ​ x 1500 x 2500 9 \dfrac{2500}{9} 9 2500 ​

= 208333.3 J

Hence, work done = 208333.3 J.

Question 18

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.

The direction of force acting on the block is perpendicular to the displacement. Hence, work done by force on the block will be zero .

The direction of force acting on the block is in the direction of displacement. Hence, work done by force on the block will be positive .

The direction of force acting on the block is opposite to the direction of displacement. Hence, work done by force on the block will be negative.

Question 19

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Acceleration in an object could be zero even when several forces are acting on it. This happens when all the forces cancel out each other i.e., the net force acting on the object is zero.

Consider the below object:

F = 0 [∵ equal and opposite forces cancel out each other]

∴ V final = V initial

Hence, if the object was initially moving, it will continue to move with uniform velocity without acceleration.

Therefore, I agree with Soni's statement.

Question 20

Find the energy in joules consumed in 10 hours by four devices of power 500 W each.

Power rating (P) = 500 W

Converting W into kW

500 W = 500 1000 \dfrac{500}{1000} 1000 500 ​ = 0.5 kW

Time (T) = 10 h

Energy consumed by each device = Power x Time taken

Energy consumed by each device = 0.5 x 10 = 5 kWh

Energy consumed by four devices = 4 x 5 = 20 kWh

So, 20 kWh = 3.6 x 10 6 x 20 = 7.2 x 10 7 J

Hence, the energy consumed = 7.2 x 10 7 J

Question 21

A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

When an object falls freely towards the ground, its potential energy decreases, and kinetic energy increases. As the object touches the ground, all its potential energy becomes kinetic energy. When the object hits the ground, all its kinetic energy gets converted into heat energy and sound energy.

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NCERT Solutions for Class 9 Science Chapter 11 Work Power and Energy

NCERT Solutions Class 9 Science Chapter 11 Work, Power And Energy – Here are all the NCERT solutions for Class 9 Science Chapter 11. This solution contains questions, answers, images, step by step explanations of the complete Chapter 11 titled Work, Power And Energy of Science taught in class 9. If you are a student of class 9 who is using NCERT Textbook to study Science, then you must come across Chapter 11 Work, Power And Energy. After you have studied lesson, you must be looking for answers of its questions. Here you can get complete NCERT Solutions for Class 9 Science Chapter 11 Work, Power And Energy in one place. For a better understanding of this chapter, you should also see Chapter 11 Work, Power And Energy Class 9 notes , Science.

Topics and Sub Topics in Class 9 Science Chapter 11 Work Power and Energy:

• Work Power and Energy
• Rate of Doing Work

These solutions are part of NCERT Solutions for Class 9 Science . Here we have given Class 9 NCERT Science Textbook Solutions for Chapter 11 Work Power and Energy

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Class 9 Science NCERT Textbook – Page 149 Q1. When do we say that work is done? Work is said to be done when a force causes displacement of an object in the direction of applied force. Q2. Write an expression for the work done when a force is acting on an object in the direction of its displacement. Work done = Force x Displacement Q3. Define 1J of work. When a force of IN causes a displacement of 1m, in its own direction the work done is said to be one joule. Q4. A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field? Work done = Force x Displacement = 140 x 15 = 2,100 J

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Class 9 Science NCERT Textbook – Page 156

Q1. What is power? Power is defined as the rate of doing work Q2. Define 1 watt of power. When a work of 1 joule is done in 1 s, the power is said to be one watt. Q3. A lamp consumes 1000 J of electrical energy in 10 s. What is its power? Given W = 1000J, t = 10s, P =? We know, P = W/t = 1000/10 = 100W Q4. Define average power. When a machine or person does different amounts of work or uses energy in different intervals of time, the ratio between the total work or energy consumed to the total time is average power.

NCERT Textbook for Class 9 Science – Page 158

Q1. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’. (a) Suma is swimming in a pond. (b) A donkey is carrying a load on its back. (c) A wind mill is lifting water from a well. (d) A green plant is carrying out photosynthesis. (e) An engine is pulling a train. (f) Food grains are getting dried in the sun. (g) A sailboat is moving due to wind energy. Work is done whenever the given conditions are satisfied: (i) A force acts on a body. (ii) There is a displacement of the body. (a) While swimming, Suma applies a force to push the water backwards. Therefore, Suma swims in the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, work is done by Seema while swimming. (b) While carrying a load, the donkey has to apply a force in the upward direction. But, displacement of the load is in the forward direction. Since, displacement is perpendicular to force, the work done is zero. (c) A wind mill works against the gravitational force to lift water. Hence, work is done by the wind mill in lifting water from the well. (d) In this case, there is no displacement of the leaves of the plant. Therefore, the work done is zero. (e) An engine applies force to pull the train. This allows the train to move in the direction of force. Therefore, there is a displacement in the train in the same direction. Hence, work is done by the engine on the train. (f) Food grains do not move in the presence of solar energy. Hence, the work done is zero during the process of food grains getting dried in the Sun. (g) Wind energy applies a force on the sailboat to push it in the forward direction. Therefore, there is a displacement in the boat in the direction of force. Hence, work is done by wind on the boat.

Q2. An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object? Since the body returns to a point which is on the same horizontal line through the point of projection, no displacement has taken place against the force of gravity, therefore, no work is done by the force due to gravity.

Q3. A battery lights a bulb. Describe the energy changes involved in the process. Within the electric cell of the battery the chemical energy changes into electrical energy. The electric energy on flowing through the filament of the bulb, first changes into heat energy and then into the light energy.

Q5. A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer. The work done is zero. This is because the gravitational force and displacement are perpendicular to each other.

Q6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why? It does not violate the law Of conservation of energy. Whatever, is the decrease in PE due to loss of height, same is the increase in the KE due to increase in velocity of the body.

Q7. What are the various energy transformations that occur when you are riding a bicycle? The chemical energy of the food changes into heat and then to muscular energy. On paddling, the muscular energy changes into mechanical energy

Q8. Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going? Energy transfer does not take place as no displacement takes place in the direction of applied force. The energy spent is used to overcome inertia of rest of the rock.

Q10. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

Q11. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer. When a satellite moves round the Earth, then at each point of its path, the direction of force of gravity on the satellite (along the radius) is perpendicular to the direction of its displacement (along the tangent). Hence, the work done on the satellite by the force of gravity is zero.

Q12. Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher. The answer is both Yes and No. Yes because when an object moves in deep space from one point to another point in a straight line, the displacement takes place, without the application of force. No, because force cannot be zero for displacement on the surface of earth. Some force is essential.

Class 9 Science NCERT Textbook – Page 159

Q13. A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer. The person does not do work because no displacement takes place in the direction of applied force as the force acts in the vertically upward direction.

Q14. An electric heater is rated 1500 W. How much energy does it use in 10 hours? Energy consumed by an electric heater can be obtained with the help of the expression, P=W/t where, Power rating of the heater, P = 1500 W = 1.5 kW Time for which the heater has operated, t = 10 h Work done = Energy consumed by the heater Therefore, energy consumed = Power × Time = 1.5 × 10 = 15 kWh Hence, the energy consumed by the heater in 10 h is 15 kWh or 15 units.

Q15. Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy? When the pendulum bob is pulled (say towards left), the energy supplied is stored in it is the form Of PE on account of its higher position. When the pendulum is released so that it starts moving towards right, then its PE changes into KE such that in mean position, it has maximum KE, and Zero PE. As the pendulum moves towards extreme right, its KE changes into PE such that at the extreme position, it has maximum PE and zero KE. When it moves from this extreme position to mean position, its PE again changes to KE. This illustrates the law Of conservation of energy. Eventually, the bob comes to rest, because during each oscillation a part of the energy possessed by it transferred to air and in overcoming friction at the point of suspension. Thus, the energy of the pendulum is dissipated in air. The law of conservation of energy is not violated because the energy merely changes its form and is not destroyed.

Q16. An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest? Kinetic energy of an object of mass m moving with a velocity v is given by the expression 1/2mv². To bring the object to rest, an equal amount of work i.e. 1/2mv² is required to be done on the object.

Q19. Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why? Yes, acceleration in an object could be zero even when several forces are acting on it. This happens when all the forces cancel out each other i.e., the net force acting on the object is zero.

Q20. Find the energy in kWh consumed in 10 hours by four devices of power 500 W each. Power rating of each device, P = 500 W = 0.50 kW Time for which each device runs, t = 10 h Work done = Energy consumed by each device (E) We know, power = Energy consumed / Time Energy consumed by each device= Power × Time E = P x t = 0.50×10 = 5 kWh Hence, the energy consumed by four devices of power 500 W each in 10 h will be 4 × 5 kWh = 20 kWh = 20 units

Q21. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy? As the object hits the hard ground, its kinetic energy gets converted into (i) heat energy (the object and the ground become slightly warm) (ii) sound energy (sound is heard when the object hits the ground) (iii) potential energy of configuration of the body and the ground (the object and the ground get deformed a little bit at the point of collision).

NCERT Solutions for Class 9 Science Chapter 11 Work, Power and Energy (Hindi Medium)

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• CBSE Notes For Class 9
• Class 9 Science Notes
• Chapter 11: Work And Energy

Work And Energy Class 9 CBSE Notes - Chapter 11

According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 10.

CBSE Class 9 Work and Energy Notes

Introduction to work and energy.

Chapter Summary Video

Work done on an object is defined as the product of the magnitude of the force acting on the body and the displacement in the direction of the force. W = F.s. The SI unit of force is Newton.

If a force acting on a body causes no displacement, the work done is 0. For example, pushing a wall.

The force component F cos θ gives the component of force along the direction in that the body is displaced. Cos θ is the angle between the force vector and displacement vector.

To know more about Work, visit here .

Energy is defined as the ability to do work. Its unit is the same as that of work. Energy is a scalar quantity.

SI unit of energy or work = Joule (Nm) or K g m 2 s − 2 .

Forms of Energy

Energy has different forms: Light, heat, chemical, electrical or mechanical. Mechanical energy is the sum of (i) Kinetic energy (K.E) (ii) Potential energy (P.E)

To know more about Energy and Its Types, visit here .

Kinetic Energy

Objects in motion possess energy and can do work. This energy is called Kinetic Energy.

When two identical bodies are in motion, the body with a higher velocity has more KE.

To know more about Kinetic Energy, visit here .

Work-Energy Theorem

The work-energy theorem states that the net work done by a moving body can be calculated by finding the change in KE.

⇒ W  n e t  =  K E  f i n a l  − KE  initial

⇒  W n e t =  \(\begin{array}{l}\frac{1}{2}\end{array} \) m [ v 2 − u 2 ]

To know more about Work-Energy Theorem, visit here .

Factors Affecting Kinetic Energy

Potential energy.

Energy can get stored in an object when work is done on it.

For example, stretching a rubber string. The energy that is possessed by a body by virtue of its configuration or change in position is known as Potential Energy.

The potential energy of an object at a height

When an object is raised to a certain height, work is done against gravity to change its position. This energy is stored as Potential Energy.

⇒W = F.s

⇒F = ma In the case of increasing the height, F = mg Therefore, W (P.E) = mgh ⇒ Δ P E = m g ( h  f i n a l − h  i n i t i a l )

For more information on Kinetic and Potential Energy, watch the below video

To know more about Potential Energy, visit here .

Law of Conservation of Energy

Law of conservation of energy states that energy can neither be created nor destroyed but can be transferred from one form to another. The total energy before and after the transformation remains constant.

Total energy = KE + PE

where, 1/2 mv 2 + mgh = constant

For example: consider a ball falling freely from a height. At height h, it has only PE = mgh.

By the time it is about to hit the ground, it has a velocity and therefore has K E =    \(\begin{array}{l}\frac{1}{2}\end{array} \)   m v 2 . Therefore, energy gets transferred from P E to K E , while the total energy remains the same.

To know more about the Law of Conservation of Energy, visit here .

The rate of doing work or the rate of transfer of energy is called power. It is denoted by P

SI unit is Watt ( J s − 1 ) .

Average power =  Total energy consumed/Total time taken

Commercial Unit of Power

The commercial unit of power is kWh, i.e. energy used in 1 hour at 1000 Joules/second. 1 k W h = 3.6 × 10 6 J

For more information on Work Energy and Power, watch the below videos

To know more about Power, visit here .

Frequently Asked Questions on CBSE Class 9 Physics Notes Chapter 11 Work and Energy

What is the definition of ‘kinetic energy’.

Kinetic energy is a form of energy that an object or a particle has by reason of its motion.

What is the definition of ‘potential energy’?

Potential energy is the stored energy that depends upon the relative position of various parts of a system.

What is ‘power’?

Power is the amount of energy transferred or converted per unit time.

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NCERT Solutions for Class 9 Science Chapter 10 Work and Energy

• 9th June 2023

NCERT Solutions for Class 9 Science Chapter 10 Work and Energy provides detailed answers for all in-text and exercise Questions. These solutions contain an in-depth explanation of each topic involved in the chapter. Students studying in class 9 can access these solutions for free in PDF format.

All these solutions are prepared by expert teachers and updated for the current academic session. NCERT Solutions for Class 9 Science Chapter 10 Work and Energy help students to understand the fundamental concepts given in class 9 Science textbook. We have prepared the answers to all the questions in an easy and well-structured manner. It helps students to grasp the chapter easily.

NCERT Class 9 Science Work and Energy Intext Questions (Solved)

PAGE NO. 115

Question 1: A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?

Answer: When a force F acts on an object to displace it through a distance S in its direction, then the work done W on the body by the force is given by:

Workdone = Force × Displacement W = F × S Where, F = 7 N S = 8 m Therefore, work done, W = 7 × 8 = 56 Nm = 56 J

PAGE NO. 116

Question 1: When do we say that work is done?

Answer: Work is done whenever the given conditions are satisfied:

• A force acts on the body.
• There is a displacement of the body caused by the applied force along the direction of the applied force.

Question 2: Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Answer: When a force F displaces a body through a distance S in the direction of the applied force, then the work done W on the body is given by the expression: Workdone = Force × Displacement W = F × s 

Question 3: Define 1 J of work.

Answer: 1 J is the amount of work done by a force of 1 N on an object that displaces it through a distance of 1 m in the direction of the applied force. 80

Question 4: A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Answer: Work done by the bullocks is given by the expression: Workdone = Force × Displacement W = F × d Where, Applied force, F = 140 N Displacement, d = 15 m W = 140 × 15 = 2100 J Hence, 2100 J of work is done in ploughing the length of the field.

PAGE NO. 119

Question 1: What is the kinetic energy of an object?

Answer: Kinetic energy is the energy possessed by a body by the virtue of its motion. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of the hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run windmills, etc. 

Question 2: Write an expression for the kinetic energy of an object.

Answer: If a body mass m is moving with a velocity v, then its kinetic energy  is given by the expression, 

Its SI unit is Joule (J).

Question 3: The kinetic energy of an object of mass, m moving with a velocity of 5 m s −1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

m = Mass of object   v = Velocity of the object in ms −1              Given that kinetic energy, E K = 25J

If the velocity of an object is doubled, then v = 5 × 2 = 10 ms −1 . Therefore, its kinetic energy becomes 4 times its original value, because it is proportional to the square of the velocity.  Hence, kinetic energy = 25 × 4 = 100 J.

If velocity is increased three times, then its kinetic energy becomes 9 times its original value, because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 9 = 225J.

PAGE NO.123

Question 1: What is power?

Answer: Power is the rate of doing work or the rate of transfer of energy. If W is the amount of work done in time t, then power is given by the expression, Power = Work/Time ⇒ Power = Energy/Time ⇒ P = W/T It is expressed in watt (W).

Question 2: Define 1 watt of power:

Answer: A body is said to have power of 1 watt if it does work at the rate of 1 joule in 1 s, i.e.,1W=1J/1s

Question 3: A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Answer: Given, Work done = Energy consumed by the lamp = 1000 J  Power is given by the expression, Power=Work done /Time ⇒ P = 1000/10 ⇒ P = 100 W

Question 4: Define average power.

Answer: A body can do different amount of work at different time intervals. Hence, it is better to find average power. Average power is obtained by dividing the total amount of work done in the total time taken to do this work.

Average Power =Total work done / Total time taken

CBSE Class 9 Science Work and Energy Exercise Questions and Answers

Question 1: Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’. (a) Suma is swimming in a pond. (b) A donkey is carrying a load on its back. (c) A wind-mill is lifting water from a well. (d) A green plant is carrying out photosynthesis. (e) An engine is pulling a train. (f) Food grains are getting dried in the sun. (g) A sailboat is moving due to wind energy.

(a) Suma is swimming in a pond.   Suma is doing work as she is able to move herself by applying force with the movement of her arms and legs in the water.

(b) A donkey is carrying a load on its back. Donkey is not doing any work (in the sense of physics) as the weight he is carrying (the direction of force) and displacement are perpendicular to each other.

(c) A windmill is lifting water from a well.   Windmill is lifting water from a well and doing work against gravity.

(d) A green plant is carrying out photosynthesis. No force and displacement are present here, so work done is zero.

(e) An engine is pulling a train. During the pulling a train, engine does the work against the friction, present between the railway track and wheels. 

(f) Food grains are getting dried in the sun.   During the drying the grains, there is no force as well as displacement is present. So, no work is done.

(g) A sailboat is moving due to wind energy. Work is done by the wind as it moves the sailboat towards the direction of the force (force of blowing air).

Question 2: An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Answer: When the object moves upwards, the work done by gravity is negative (as the direction of gravitational force is towards the Earth’s centre) and when the object comes downwards, there is a positive work done. So, the total work down is zero throughout the motion.

Question 3: A battery lights a bulb. Describe the energy changes involved in the process.

Answer 3: Battery converts chemical energy into electrical energy. This electrical energy is further converted into light and heat energy. 

Question 4:  Certain force acting on a 20 kg mass changes its velocity from 5ms –1 to 2ms –1 . Calculate the work done by the force.

Answer: Mass of the body = 20 kg Initial velocity = 5 m/s Final velocity = 2 m/s

Question 5: A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.

Answer: The work down by the gravitational force acting on the body is zero because the direction of force is vertically downward and the displacement is horizontal i.e. force and displacement are perpendicular to each other. 

Question 6: The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Answer: The potential energy of freely falling object decreases and its kinetic energy increases (as its velocity increases) progressively. So, in this way the total mechanical energy (Kinetic energy + potential energy) remains constant. Thus, the law of conservation of energy is not violated.

Question 7: What are the various energy transformations that occur when you are riding a bicycle?

Answer: The muscular energy of the cyclist is converted into kinetic (rotational) energy of wheels of the cycle which is further converted into kinetic energy to run the bicycle.

Question 8:  Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Answer: When we push the rock and fail to move it. Some of our energy is absorbed by the rock in the form of potential energy and the rest of our energy is goes to the environment through our muscles and the surface between the rock and out hand.

Question 9: A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

Answer: Energy consumed in one month = 250 unit = 250 kWh = 250 kW×h = 250×1000W×3600s = 900,000,000 J = 9 × 10 8  J.

Question 10: An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

Answer: We know that, potential energy = mgh Where, m = 40 kg g = 9.8 m/s 2 h = 5 m So, the potential energy = 40 × 9.8 × 5 J = 1960 J According to law of conservation of energy, the total mechanical energy (Kinetic and potential energy) of an object remains constant.  Therefore, when the object is half-way down, its potential energy become half the original energy and remaining half converted into kinetic energy. Hence, the kinetic energy = ½ (1960) J = 980 J

Question 11: What is the work done by the force of gravity on a satellite moving round the Earth? Justify your answer.

Answer: When a satellite moves around the Earth, the displacement in short interval is along the tangential direction and the force (gravitational force) is towards the centre of the Earth. Since, the force and displacement are perpendicular to each other, the work done by gravitational force is zero.  

Question 12: Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.

Answer: Yes, it is true. There may be displacement in the absence of force. We know that, F = ma, In the absence of force, F = 0, then ma = 0 

If a = 0, the object is either at rest or in a state of uniform motion in a straight line. In case the object is moving in a straight line, there must be displacement. So, in the absence of force, there may be displacement in the object. 

Question 13: A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.

Answer : The person holding a bundle of hay get tired because his muscular energy is converting into thermal energy. There is no displacement at all, so he had no work as workdone = Force × displacement.

Question 14: An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Answer: We know that, Energy = Power × time Here, Power = 1500 W Time = 10 hours = 10 × 60 × 60 seconds = 36000 seconds Therefore,  The energy used by heater  = Power × time = 1500 × 36000 J = 54000000 J = 5.4 × 10 7 J

Question 15: Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?

Answer : In the given pendulum, there are three cases of points to be discussed.

At the points B and C, the pendulum bob is at its maximum height, so its potential energy is maximum and kinetic energy is zero. In this way the total mechanical energy remains constant. At the point A, the pendulum bob is at its lowest point, total potential energy is converted into kinetic energy. Now the kinetic energy is maximum and potential energy is zero. Once again the total mechanical energy remains constant

Question 16: An object of mass,   is moving with a constant velocity, . How much work should be done on the object in order to bring the object to rest?

Question 17: Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?

The kinetic energy of the car, when it comes to rest = 0 J Workdone on object  = change in kinetic energy = 208333.3 − 0 = 208333.3 J Hence, the work required to stop the car is 208333.3 J.

Question 18: In each of the following a force, F is acting on an object of mass, m . The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.

Answer: In the first case, the force and displacement are perpendicular to each other, so work done is zero. In the second case, the force and displacement are in the same direction, so the work done is positive. In the third case, the force and displacement are in the opposite direction, so the work done is negative.

Question 19: Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Answer 19: If the resultant force acting on a body in different directions is zero, then the acceleration will be zero.   We know that,                               F = ma, In the net force is zero,             F = 0 ⇒ ma = 0  ⇒ a = 0  [as m ≠ 0]

Question 20: Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.

Answer: The power of four devices = 4 × 500 W = 2000 W Time = 10 hours Therefore, the energy consumed = power × time = 2000 × 10 Wh = 20000 Wh = 20 kWh  = 20 units   [1 unit = 1 kWh]

Question 21: A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

Answer: When a freely falling body eventually stops on reaching the ground, its kinetic energy gets converted into heat energy (as the body and ground become warm due to collision), sound energy and into potential energy (due to change of shape or deformation).