NCERT Solutions for Class 9 Science Chapter 11 Work and Energy

Updated on Jul 24, 2020 - 8:35 p.m. IST by Priyanka kumari

NCERT solutions for class 9 science chapter 11 Work and Energy: These solutions will give you a better understanding of concepts present in this chapter through various real life examples, such as, when a person is holding a bucket of water for 10 minutes, we can say, he is physically doing work but according to Physics he is not doing work because bucket has no displacement. The CBSE NCERT solutions for class 9 science chapter 11 Work and Energy will clear your concept of work in physics with a condition to say that a work has been done by a force. The solutions for NCERT class 9 science chapter 11 Work and Energy will provide you with an overview of the complete chapter and NCERT solutions can help you walk through other chapters and other subjects also if you wish to study. Below you can check some questions and answers which will help you to clear your concepts.

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When work is said to be done?

Work is said to be done when a force is applied and due to the force, the objects have a displacement.

If F is the constant force acting on the object and S is the displacement then work W= FS. The unit of work is Joule and 1 Joule= 1N m.

An important topic of NCERT Solutions for Class 9 Science Chapter 11 is :

Energy: Energy is the capability of doing work. The unit of energy is the same as that of energy.

Kinetic Energy: The kinetic energy of an object moving with a certain velocity is the work done on the object to acquire that velocity. Suppose initial velocity of an object is u, by applying a constant force we changed the velocity to v. Let the acceleration due to force be a and S be the displacement then work done.

$\\W=FS\\s=\frac{v^2-u^2}{2a}\\W=mas\\W=ma\frac{v^2-u^2}{2a}\\W=\frac{1}{2}m(v^2-u^2)$

This work done is known as kinetic energy

If the body starts from rest then u=0 then the kinetic energy $=\frac{1}{2}mv^2$

Potential Energy: Potential energy of an object is the energy possessed by an object due to its position. The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity. Suppose the mass of the object raised is m and the height to which object is raised is h then,

Potential Energy = mgh

One of the main topics of NCERT Solutions for Class 9 Science Chapter 11 Work and Energy is given below:

Work-Energy Theorem: When energy gets transformed total energy gets conserved. That is energy can neither be created nor be destroyed. For example, When we open a tape the potential energy stored in the water tank at height is converted to the kinetic energy of the water. When an electric fan is working electrical energy is converted to mechanical energy and certain energy are lost in the form of heat.

Important Topics of NCERT Solutions for Class 9 Science Chapter 11 Work and Energy:

11.1 Work

11.1.2 Scientific Conception Of Work

11.1.3 Work Done By A Constant Force

11.2 Energy

11.2.1 Forms Of Energy

11.2.4 Potential Energy Of An Object At A Height

11.2.5 Are Various Energy Forms Interconvertible?

11.2.6 Law Of Conservation Of Energy

11.3 Rate Of Doing Work

11.3.1 Commercial Unit Of Energy

Power: The rate of doing work is Power. Power measures the speed of work done, that is, how fast or slow work is done. If an agent does a work W for time t, then power

$P=\frac{W}{t}$

The unit of power is watt.

Commercially electric bills are calculated in terms of unit and 1 unit= 1Kilo Watt-hour

$1KWH=1\times1000W\times60\times60sec=3.6\times10^6\ joule$

That is if equipment with power rating 1 KW is operated for 1 hour then it consumes 1 unit of energy.

NCERT solutions for class 9 science chapter 11 Work and Energy: Solved In-text Questions-

Topic 11.1 Work

Q1. A force of $7\; N$ acts on an object. The displacement is, say $8\; m$ , in the direction of the force (Fig. 11.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case? Let us take it that the force acts on the object through the displacement. What is the work done in this case?

Answer:

The work done is given by :

$W\ =\ F.s$

or $=\ 7\times 8$

$=\ 56\ J$

Thus the work done by the force is 56 J.

NCERT solutions for class 9 science chapter 11 Work and Energy

Topic11.1.3 Work done by a constant force

Q1 . When do we say that work is done?

Answer:

The work is done when an applied force causes some displacement. Further, if the direction of the displacement is in the direction of the force then work done by that force is positive and if the displacement is in the opposite direction then the work done is negative.

Q2. Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Answer:

The force is acting in the direction of displacement so the work done by the force is positive.

The work done is given by :

$W\ =\ F.s$ ( Here s is the displacement.)

Q3. Define $1\; J$ of work.

Answer:

1 J of work is defined as the 1 m displacement of a body due to the 1 N of force.

$W\ =\ F.s\ =\ 1\times 1\ =\ 1J$

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?

Answer:

T he work done is given by :

$W\ =\ F.s$

or $=\ 140\times 15\ =\ 2100\ Nm$

Hence the work done in ploughing the field is 2100 J.

NCERT textbook solutions for class 9 science chapter 11 Work and Energy

Topic 11.2 Energy

Q1. What is the kinetic energy of an object?

Answer:

The energy of a particle which is due to its motion is the kinetic energy.

It is given by :

$K.E.\ =\ \frac{1}{2}mv^2$

Here, m is the mass of the particle and v is the velocity.

Q2. Write an expression for the kinetic energy of an object.

Answer:

The kinetic energy of a body is given by :

$K.E\ =\ \frac{1}{2}mv^2$

Here m is the mass of the body and v is the velocity.

Q3 . 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?

Answer:

Using the formula for the kinetic energy we have :

$K.E\ =\ \frac{1}{2}mv^2$

or $25\ =\ \frac{1}{2}m\times 5^2$

or $m\ =\ 2\ Kg$

(i) Kinetic energy when velocity is doubled :

$K.E\ =\ \frac{1}{2}mv^2$

$=\ \frac{1}{2}\times2 \times 10^2$

$=\ 100\ J$

(ii) Kinetic energy when velocity is tripled :

$K.E\ =\ \frac{1}{2}mv^2$

$=\ \frac{1}{2}\times 2\times 15^2$

$=\ 225\ J$

Free NCERT solutions for class 9 science chapter 11 Work and Energy

Topic 11.3 Rate of doing work

Q1. What is power?

Answer:

Power is defined as the rate of doing work. Mathematically, it is given by :

$P\ =\ \frac{W}{t}$

The SI unit of power is a watt.

Q2. Define $1\; Watt$ of power.

Answer:

1 Watt of power is defined as the 1 joule of work is done in 1 sec.

$P\ =\ \frac{1\ J}{1\ sec}\ =\ 1\ Watt$

Q3. A lamp consumes $1000 \; J$ of electrical energy in $10 \; s$ . What is its power?

Answer:

The power is given by :

$P\ =\ \frac{W}{t}$

$=\ \frac{1000}{10}\ =\ 100\ watt$

Thus power is 100 watt.

Q4. Define average power.

Answer:

Average power is defined as the total work done in a particular time period divided by the time period.

$P_{avg}\ =\ \frac{W_{total}}{T}$

NCERT textbook solutions for class 9 science chapter 11 Work and Energy: Solved Exercise Questions-

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’.

• Suma is swimming in a pond.

• A donkey is carrying a load on its back.

• A wind-mill is lifting water from a well.

• A green plant is carrying out photosynthesis.

• An engine is pulling a train.

• Food grains are getting dried in the sun.

• A sailboat is moving due to wind energy

Answer:

(i) Work done by Suma is negative as the force and displacement are in the opposite direction.

(ii) Work done is zero as the gravity on the load is acting vertically downward whereas its displacement is in a horizontal direction.

(iii) Work done is positive as both force and displacement are in an upward direction.

(iv) Work done is zero as there is no displacement involved.

(v) Work done is positive as force is acting in the direction of the motion.

(vi) Work done is zero as there is no displacement of the grains.

(vii) Work done by wind force is positive as it supports the motion of 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?

Answer:

We can see that there is no net displacement in the vertical direction. It has only horizontal displacement. Thus work done by vertical forces on the stone will be zero.

The force of gravity is acting vertically downward, thus the work done by it is zero.

Q3. A battery lights a bulb. Describe the energy changes involved in the process.

Answer:

The chemical energy stored in a battery is transformed into the heat energy which glows the filament of the bulb. Further, the heat energy is converted into light energy.

Q4. Certain force acting on a mass changes its velocity from $5\; m\; s ^{-1}$ to $2\; m\; s ^{-1}$ . Calculate the work done by the force.

Answer:

By the equations of motion, we can write :

$v^2\ =\ u^2\ +\ 2as$

$s\ =\ \frac{v^2\ -\ u^2}{2a}$

$=\ \frac{2^2\ -\ 5^2}{2a}\ =\ \frac{-21}{2a}\ m$

And the work done is :

$W\ =\ F.s$

$=\ 20a\times \frac{-21}{2a}$

$=\ -210\ J$

Thus work done is - 210 J.

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

Answer:

The displacement of the object is horizontally on the table. We know that the gravitational force is acting in a downward direction. There is no displacement vertically.

Thus the work done by the gravitational force is zero .

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

Answer:

No, there is no violation of the law of conservation of energy. At the maximum height, the energy is in the form of potential energy. When the object is reaching the ground, its potential energy is decreasing whereas its kinetic energy is increasing (as the velocity of the object is increasing). Thus there is no loss of total energy (energy transformation may take place ).

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

Answer:

The muscular energy of a person is transformed in the form of mechanical energy which helps to rotate the wheel of bicycles.

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?

Answer:

Since the rock doesn't move thus net displacement is zero. As a result, the work done by the force is zero.

The energy which we apply on the rock gets transformed in the form of heat.

Q9. A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

Answer:

We know that 1 unit of energy is given by : 1 unit = 1 KWh.

Also, $1\ KWh\ =\ 3.6\times 10^6\ J$

Thus 250 units in joule is given by :

$E\ =\ 250\times 3.6\times 10^6\ =\ 9\times 10^8\ J$

Q 10. An object of mass 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:

The potential energy of an object of mass m and at the height at h is given as = mgh

The potential energy of the given object is :

$P.E.\ =\ mgh\ =\ 40\times 10\times 5\ =\ 2000\ J$

The potential energy is being converted into the K.E..

Thus at half-way kinetic energy of the object is :

$K.E.\ =\ \frac{P.E.}{2}\ =\ \frac{2000}{2}\ =\ 1000\ J$

Q11. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

Answer:

The work done by the gravitational force is zero. This is because the satellite is moving in a circular orbit. Thus the direction of displacement of the satellite is perpendicular to the force of gravity. Hence work done 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.

Answer:

This depends on the initial state of the object. If the object is in motion initially, then it will continue to be in this state as no external force is acting. But if the object is at rest initially then the object can't move without external force.

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.

Answer:

Since there was no displacement thus the work done by man is zero . This work should not be considered relatable to the term 'work' we use in daily life.

Due to upholding the weight against gravity led the man to get tired.

Q14. An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Answer:

The relation between energy and power is given by :

Energy = Power $\times$ Time

Thus the energy used in 10 hours is :

$E\ =\ 1500\times 10\ =\ 15000\ Wh\ =\ 15\ KWh$

Hence the energy used by the heater is 15 KWh.

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?

Answer :

In the above figure, point B is the mean position about which the bob rotates.

When the bob is released from point C, it attains some velocity while moving down (up to B) and deaccelerated and stops at point A.

Thus point A and point C are the maximum height points. And the velocity of the bob at point B will be maximum.

The total energy at point A and point C is only the potential energy as its velocity at these points is zero. And at point B as the height of bob is zero thus the total energy is just the kinetic energy.

Thus in this manner, the conservation of energy takes place (by transforming in some other form.).

The eventually comes to rest due to the air resistance. It deacceleration the motion of bob. (as it is a frictional force.)

There is no violation of the energy conservation law as some amount of energy is converted in the form of heat.

Q. 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?

Answer:

At this moment the energy of the object is :

$K.E.\ =\ \frac{1}{2}mv^2$

Thus in bringing the object to rest the work needed is : $=\ K.E.$

$W\ =\ \frac{1}{2}mv^2\ J$

Q17. Calculate the work required to be done to stop a car of moving at a velocity of ?

Answer:

Firstly convert the velocity in SI unit.

$v\ =\ 60\times \frac{5}{18}\ =\ \frac{50}{3}\ m/s$

Thus the work done to stop the car is equal to the kinetic energy of the car.

$W\ =\ \frac{1}{2}mv^2\ =\ \frac{1}{2}\times 1500\times \left ( \frac{50}{3} \right )^2$

$=\ 208333.33\ J\ or\ 208.33\ KJ$

Q18. 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:

(i) In first case, the force acting on the object is perpendicular to the displacement of the body. Thus the work done by the force is zero.

(ii) In this, the force is in direction of the displacement so the work done is positive.

(iii) In this case, the direction of force and displacement are opposite to each other. Thus the work done by the force is negative.

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?

Answer:

Yes. If all the external forces acting on the body balances each other then the net force acting on the object is zero. Thus the acceleration produced is zero even if forces are acting on the body. The necessary condition for the zero acceleration is that the net force acting on the body should be zero.

Q 20. Find the energy in $kW\; h$ consumed in $10$ hours by four devices of power $500 \; W$ each.

Answer:

The energy consumed by 1 device is given by :

$E\ =\ 500\times 10\ =\ 5000\ Wh$

Thus the energy consumed by 4 devices is $=\ 4\times 5000\ =\ 20\ KWh$

Q21. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

Answer:

When an object is falling from a height, there is an increase in the kinetic energy of the object. Thus just before hitting the ground the kinetic energy of the object is very high. But after hitting the ground its velocity comes to zero and thus the kinetic energy becomes zero. The kinetic energy is transformed into other forms of energy such as heat, sound or some deformation on the ground.

NCERT Solutions for Class 9 Science- Chapter Wise

Chapter No.	Chapter Name
Chapter 1	NCERT solutions for class 9 science chapter 1 Matter in Our Surroundings
Chapter 2	NCERT solutions for class 9 science chapter 2 Is Matter Around Us Pure
Chapter 3	NCERT solutions for class 9 science chapter 3 Atoms and Molecules
Chapter 4	NCERT solutions for class 9 science chapter 4 Structure of The Atom
Chapter 5	NCERT solutions for class 9 science chapter 5 The Fundamental Unit of Life
Chapter 6	NCERT solutions for class 9 science chapter 6 Tissues
Chapter 7	NCERT solutions for class 9 science chapter 7 Diversity in Living Organisms
Chapter 8	NCERT solutions for class 9 science chapter 8 Motion
Chapter 9	NCERT solutions for class 9 science chapter 9 Force and Laws of Motion
Chapter 10	NCERT solutions for class 9 science chapter 10 Gravitation
Chapter 11	NCERT solutions for class 9 science chapter 11 Work and Energy
Chapter 12	NCERT solutions for class 9 science chapter 12 Sound
Chapter 13	NCERT solutions for class 9 science chapter 13 Why Do We Fall ill?
Chapter 14	NCERT solutions for class 9 science chapter 14 Natural Resources
Chapter 15	NCERT solutions for class 9 science chapter 15 Improvement in Food Resources

NCERT Solutions for Class 9 - Subject Wise

NCERT Solutions for Class 9 Maths

NCERT Solutions for Class 9 Science

How to Use NCERT Solutions for Class 9 Science Chapter 11 Work and Energy?

If you want to score well in your exam then the practice should be the only way which will help you to get high scores in the CBSE class 9 science exam.
While preparing for the exam, you must go through the NCERT solutions for class 9 science chapter 11 Work and Energy. Since in your exams, there will be direct questions from the NCERT textbook.
The solutions are created by experienced science faculty as per the CBSE guidelines.

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NCERT Solutions for Class 9 Science Chapter 11 Work and Energy

Important Topics of NCERT Solutions for Class 9 Science Chapter 11 Work and Energy:

NCERT solutions for class 9 science chapter 11 Work and Energy: Solved In-text Questions-

NCERT textbook solutions for class 9 science chapter 11 Work and Energy: Solved Exercise Questions-

NCERT Solutions for Class 9 Science- Chapter Wise

NCERT Solutions for Class 9 - Subject Wise

How to Use NCERT Solutions for Class 9 Science Chapter 11 Work and Energy?

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