# Class 9 Science Chapter 11 Work and Energy

NCERT Class 9 Science Chapter 11 Work and Energy Question Answer to each chapter is provided in the list so that you can easily browse throughout different chapters Class 9 Science Chapter 11 Work and Energy and select need one.

## NCERT Class 9 Science Chapter 11 Work and Energy

Also, you can read the SCERT book online in these sections Solutions by Expert Teachers as per SCERT (CBSE) Book guidelines. These solutions are part of SCERT All Subject Solutions. Here we have given SCERT Class 9 Science Chapter 11 Work and Energy Solutions for All Subjects, You can practice these here.

### Work and Energy

Chapter – 11

GENERAL SCIENCE

Textbook Page No. 148.

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

Ans. Force, F = 7 N

Displacement, s = 8 m

∴ Work done, W = Fs

= 7 N × 8 m

= 56 N m

= 56 j.

Textbook Page No. 149

1. When do we say that work is done?

Ans: When do we say that work is done? Ans. Work is said to be done when force applied on an object shows the displacement in that object. W = F x s.

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

Ans. Work done = Force × Displacement, when the force is acting on the object in the direction of displacement.

3. Define 1 J of work.

Ans. One joule (J) is the amount of work done on an object when a force of 1 N displaces it by 1 m along the distance line of action of the force.

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?

Ans.

Textbook Page No. 152

1. What is the kinetic energy of an object?

Ans. Kinetic energy is the energy an object has because of its motion. If we want to accelerate an object, then we must apply a force. Applying a force requires us to do work. After work has been done, energy has been transferred to the object, and the object will be moving with a new constant speed.

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

Ans. The expression for the kinetic energy possessed by an object of mass, m and moving with a uniform velocity, vis given by,

.

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?

Textbook Page No. 156

1. What is power?

Ans.

2. Define 1 watt of power.

Ans. Watt is a SI unit of power which is defined as the energy per unit time i.e P=Et. Hence one watt is defined as one joule of energy used in one second.

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

Ans.

4. Define average power.

Ans. A body may perform work at different rates at different intervals of time. The average power is defined as the ratio of total work done or total energy consumed by the total time taken.

EXERCISES

Textbook Page No. 158

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.

Ans: Work is done as Suma is displacing water in the backward direction while swimming.

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

Ans: No work is done because the donkey exerts a vertically upward force on the load, but the displacement of the load is in the horizontal direction.

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

Ans: Work is done as there is displacement of water against the force of gravity.

(D) A green plant is carrying out photosynthesis.

Ans: No work is done as there is no displacement of leaves during photosynthesis.

(E) An engine is pulling a train.

Ans: Work is done as the train is being displaced in the direction of the applied force.

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

Ans: No work is done as the food grains are not displaced.

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

Ans: Work is done as there is displacement of the sail boat.

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?

Ans. The initial and the final positions on the path of the object thrown at a certain angle to the ground lie on the some horizontal plane, the displacement of the object is in the horizontal direction. The force of gravity on the object acts vertically downwards, so no work is said to be done.

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

Ans. A battery has chemicals inside it which can react with each other and generate electricity through the flow of electrons. Hence when a battery is connected with a bulb, its chemical potential energy is converted into electrical energy. This electrical energy is then transferred to the circuit containing the bulb.

The electrical energy is then converted into light and heat energy.

This is shown below:

Chemical energy → Electrical energy → Light energy → Heat energy.

4. Certain force acting on a 2 kg mass changes its velocity from 5 m s⁻¹ to 2 m s⁻¹. Calculate the work done by the force.

Ans.

The negative sign ‘_’ shows that the force acts in a direction opposite to the direction of the motion of the mass, i.e. the force is retarding.

Hence the magnitude of the work done by the force is 210 J.

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 force? A and B is horizontal, what is the work done on the object by the gravitational Explain your answer.

Ans. As shown in the figure, the mass m is lying at point A on the table at a height h from the ground.

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

Ans. No, if the potential energy of a freely falling object decreases progressively, it does not violate the law of conservation of energy. it does not violate the law of conservation of energy. This is because as the object fails, its potential energy gradually converts into kinetic energy.Let us consider an object placed at position X at a certain height from the ground. At this position, the object possesses only potential energy and the kinetic energy is zero. Now the object is allowed to fall freely. Its potential energy gradually converts into kinetic energy. At position Y, at some distance from X, the object possesses some potential energy and some kinetic energy. But the sum of potential energy and kinetic energy of the object at position Y is equal to the potential energy of the object at the position X. At position 2, L.e. when the object just touches the ground, the whole of the potential energy of the object gets converted into kinetic energy. Therefore at position Z, the object possesses only kinetic energy and its potential energy is zero. But the kinetic energy of the object at position Z is equal to the potential energy of the object at position X.

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

Ans. The various energy transformations that occur when you are riding a bicycle are given below: The rider’s muscular energy is converted to heat energy and the bicycle’s kinetic energy while riding a bicycle. The rider’s body is heated by heat energy. The bicycle is propelled in a circular motion by kinetic energy

Transformatioms occur are mentioned below:

Muscular → Kinetic energy → Heat energy.

The total energy remains conserved during this transformed.

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?

Ans. No, there is no transfer of muscular energy to stationary rock in this case. Also, there is no loss of energy as our muscular energy transfers to heat energy as a result of which our body becomes hot.

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

Ans. Therefore, 250 units of energy = 250×3.6×106 = 9×108J.

A certain household has consumed 250 units of energy during a month.

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.

Ans.

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

Aus. When an object moves in a direction different from the direction of the force, the work done by the force F is given by:

W = Fs cosθ, where θ is the angle between the direction of the force (F) and the direction the motion (s).

When an artificial satellite revolves around the earth due to the force of gravity, the motion of the satellite is in the tangential direction, whereas the force of gravity acts on it directed towards the centre.

Here  0= 90o

∴ work done = Fs cos 90o

= Fs x 0

= 0

Hence , the work done is zero.

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.

Ans. No, there cannot be displacement of an object in the absence of any force acting on it.

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

Ans. The person does not move from his position (no displacement) in spite of applying force on the bundle. Therefore, no work is done.

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

Ans. Given that,

Power of the heater = 1500 W = 1.5 kW

Time used = 10 h

∴ Energy = Power × time used

= 1.5 kW × 10h = 15 kWh = 15 units

∴ The energy consumed by the heater is 15 units.

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?

Ans. A simple pendulum is an arrangement in which a small mass is suspended by a long and fine thread from some rigid support. The mass, in general, is a small metallic ball and is called the bob of the pendulum. The bob is free to swing at the lower end.

The figure given below shows a simple pendulum:

The bob is pulled to one side to position C. At this position, it possesses potential energy due to its higher position with respect to position B and then its kinetic energy is zero. As the bob is released from rest at position C, it starts swinging between the extreme positions C and A. As soon as the bob is released from position C, its potential energy decreases whereas its kinetic energy begins to increase. At the mean position B, it possesses only kinetic energy but no potential energy. When the bob reaches the other extreme position A, it stops momentarily. Therefore, its energy becomes totally potential. At the intermediate positions like D and E, its energy is partly potential and partly kinetic.

Thus, we see that at the extreme positions Cand A, the energy of the bob is totally potential and at the mean position B, the energy is totally kinetic. At all other intermediate positions, its energy is partly potential and partly kinetic. But at any instant of time the total energy of the swinging pendulum remains the same (conserved).

The bob eventually comes to rest because of the friction at the point of suspension of the pendulum and the friction of air on the swinging pendulum.

The mechanical energy of the swinging pendulum is converted into heat energy gradually. Therefore, there is no violation of the law of conservation of energy.

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?

Ans. Given that, Mass of the object = m

Initial velocity = v

The negative sign ‘_’ shows that the direction of the force applied to do the work is opposite to the direction of the motion of the object.

Thus, the magnitudeof wark on the object the to rest is ½ mv2.

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

Ans.

The negative sign  shows that the direction of the force applied to do the work is opposite to the direction of the motion of the car.

Thus, the magnitude of work done is 208333.3 J.

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.

Ans.

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?

Ans. Yes, Soni is correct. When the resultant of several forces acting on an object is zero, the acceleration of the object can be zero.

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

Ans.

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

Ans. 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 gets converted into kinetic energy. As the object hits the hard ground, all its kinetic energy gets converted into heat energy and sound energy.

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