In this chapter, you will understand the scientific conception of work and different types of energy. The chapter includes many numerical also so it will require your mathematical skills as well. You need to practice the questions daily. These numerical are really very easy if you understand them and practice them on a daily basis.

**NCERT Solution** Class 9 Science of Chapter 11 includes all Intext exercises 1, 2, 3, 4, and an **NCERT** exercise for you. The NCERT exercise includes solutions to all 21 questions given in your NCERT Textbook. All these questions are based on your numerical abilities and application skills.

## NCERT Solution for Class 9 Science Chapter 11 : Work and Energy Ex 11.1

**Exercise 11.1.3**

**Question 1.**

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

**Solution**: Refer pdf.

**Exercise 11.2**

**Question 1.**

**When do we say that work is done?**

**Solution**:

Work is done by a force on an object if a force acts on the object and the object is displaced from its original position.

**Question 2.**

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

**Solution**:

If F is the constant force acting in the direction of displacement s, then work done by the force, i.e.

Refer pdf.

**Question 3.**

**Define 1 J of work.**

**Solution**:

The amount of work done when a force of l N moves a body through a distance of l m in the direction of the force is called 1 Joule.

**Question 4.**

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

**Solution**: Refer pdf.

**Exercise 11.2.2**

**Question 1.**

**What is the kinetic energy of an object?**

**Solution**:

Definition:

Kinetic energy of an object is the energy possessed by it due to its motion.

Notation: KE

Formula: Refer pdf.

**Question 2.**

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

**Solution**: Refer pdf.

**Question 3.**

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

**Solution**: Refer pdf.

**Exercise 11.3**

**Question 1.**

**What is power?**

**Solution**: Refer pdf.

**Question 2.**

**Define 1 watt of power.**

**Solution**: Refer pdf.

**Question 3.**

**A lamp consumes 1000 J of electrical energy in 10 5. What is its power?**

**Solution**: Refer pdf.

**Question 4.**

**Define average power.**

**Solution**:

If power of a person/machine varies with time, then his average power may be obtained by dividing total energy consumed (or total work done) by toe total time taken.

Average power = Total energy consumed (or total work done) / Total time

**Exercise 1**

**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’.**

**1) Suma is swimming in a pond.**

**2) A donkey is carrying a load on its back.**

**3) A wind-mill is lifting water from a well.**

**4) A green plant is carrying out photosynthesis.**

**5) An engine is pulling a train.**

**6) Food grains are sun dried.**

**7) A sailboat is moving due to wind energy.**

**Solution**:

1) Sum is doing work. She is applying force to move horizontally.

2) Donkey is not doing any work. Here, the displacement and the force are at 90°.

3) Work is done by the windmill. The water is lifted against force of gravity.

4) No work is done by a green plant during photosynthesis.

5) The engine applies a pulling force on the train, and the train moves in the direction of this force. Therefore, engine is doing work.

6) During drying of food grains in the sun no work is done.

7) Work is done by the air. The sailboat moves in the direction of the force exerted by wind.

**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?**

**Solution**:

Since initial and the final positions on the path of the object thrown at a’ certain angle to the ground lie on the same 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.

**Question 3.**

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

**Solution**:

A battery converts chemical energy into electrical energy. This electrical energy is converted into light energy as the bulb is lighted up, i.e., the sequence of energy changes is as follows.

Chemical energy –>Electrical energy –>Light energy –> Heat energy

**Question 4.**

**Certain force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s. ****Calculate the work done by the force.**

**Solution**: Refer pdf.

**Question 5.**

**A mass of 10 kg is at a point A on a table. It is moved to a point B. lf the line**

**joining A and B is horizontal, what is the work done on the object by the**

**gravitational force? Explain your answer.**

**Solution**:

Here, the mass of 10 kg is moved horizontally from point A to B. The force of gravity acts vertically downwards. Thus, the mass moved in a direction at right angle to the force of gravity.

So, Work done on the object by the force of gravity = FScos = FScos90° = 0

Therefore, no work is done by the force of gravity on the object.

**Question 6.**

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

**Solution**:

As the potential energy of the freely falling object decreases, its kinetic energy increases on account of an increase in its velocity. The sum total of the potential energy and the kinetic energy of the object during its free fall remains the same, i.e., the total mechanical energy (potential energy + kinetic 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?**

**Solution**:

At the time of riding a bicycle the rider applies force and rotates the paddle. In the process, he is doing mechanical work. As a result of this work, wheels of bicycle start moving. Thus, mechanical Work is converted into kinetic energy. A part of this kinetic energy is used to overcome friction force acting on tyres by the road. This work done against friction is transformed into heat energy.

**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?**

**Solution**:

No transfer of energy takes place when we push a huge rock unsuccessfully.

The energy is spent for the physical activity of muscles.

**Question 9.**

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

**Solution**:

We know that one household unit of energy

=1kWh =3.6×106

As the given household has consumed 250 units of energy, hence the energy spent in Joules.

E=250kw h=250 ×3.6 x 106 J = 900×106 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.**

**Solution**: Refer pdf.

**Question 11.**

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

**Solution**:

When a satellite moves around the Earth, its displacement in a short interval is along the tangent to the circular path of the satellite.

The gravitational force acting on the satellite due to the displacement and the force are perpendicular to each other. Thus work done by the force of gravity on the satellite 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.**

**Solution**:

Yes. In the absence of any force on the object, i.e. F = 0, ma = 0 (as F = ma). Since m not equal to 0, a =0, In such a case, the object is either at rest or in a state of uniform motion in a straight line. In the latter case, there is a displacement of the object without 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.**

**Solution**:

The person holding a bundle of hay over his head has not done any work. In present case, the person is applying a force to hold the bundle but there is no displacement of the bundle of hay. In the absence of displacement, the work done by the person is zero.

**Question 14.**

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

**Solution**:

Here,

Power of the electric heater, P = 1500 W Time for which it is used, t= 10 h

W = P x t = 1500W ×10 h = 15000 Wh = 15KW h = 15 units

Energy consumed = 15× 3.6×106 J

= 54 × 106 J

**Question 15.**

**Illustrate the law of conservation of energy by discussing the energy charges ****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?**

**Solution**:

Consider a pendulum bob suspended from a rigid support 0. Let the bob be taken from its mean position A to one side up to point B. Initially at B the bob is at rest. It means its kinetic energy isb zero. But as shown in Fig. I I2, as compared to mean position A the bob at B is at a height h. It means that it possesses potential energy.Let the bob is allowed to oscillate. As it comes towards the mean position. A, the potential energy of bob gradually decreases. In accordance with conservation law of energy, its kinetic energy progressively increases. At mean position A, whole energy of pendulum bob is kinetic. As bob urther moves from A to C, the kinetic energy progressively decreases and the potential energy, in turn, increases. At extreme position C whole energy of bob is again potential. Thus, during its motion the energy of pendulum bob way be converted from potential energy to kinetic energy or reverse but total sum of energy remains conserved. The bob eventually comes to rest on account of force of friction and resistance due to air medium. Energy of bob is gradually dissipated to overcome air resistance and is converted into heat energy. Hence, even now the conservation law of energy is not violated.

**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?**

**Solution**: Refer pdf.

**Question 17.**

**Calculate the work required to be done to stop a car of 1500 Kg moving at a**

**velocity of 60 Km/h?**

**Solution**: Refer pdf.

**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.**

**Solution**:

(a) Here, the force acts at right angle to the displacement. So, work done by the force = 0

(b) Here, the direction of displacement is the same as that of the force. So, Work done by the force = +ve

(c) Here, the body moves in a direction Opposite to the direction of the force. So,

Work done by the force = -ve

**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?**

**Solution**:

The acceleration of an object can be zero even if several forces are acting on it provided the resultant force (F) is zero. As F = 0, ma = 0 and accordingly a = 0 (as m ¢ 0).

**Question 20.**

**Find the energy in kW h consumed in 10 hours 5 by four devices of power 500 ****W each.**

**Solution**:

Power of each device = 500 W

Number of devices = 4

Duration of use = 10 h

Total energy consumed 5 4 × 500 W × 10 h = 20000 Wh = 20 kW h

**Question 21.**

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

**Solution**:

When a freely falling body eventually stops on reaching the ground, its kinetic energy appears in the form of:

(1) Heat (the body and the ground become warmer due to collision).

(2) Sound (produced due to collision of the body with the ground).

(3) Potential energy of configuration of the body and the ground (the body may be deformed and the ground may be depressed at the place of collision).