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

**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.

This work done is known as kinetic energy

If the body starts from rest then u=0 then the kinetic energy

**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

*The unit of power is watt. *

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

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 **

** Answer: **

The work done is given by :

or

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.

** 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 :

( Here s is the displacement.)

** Q3. ** Define of work.

** Answer: **

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

** Q4 ** . A pair of bullocks exerts a force of on a plough. The field being ploughed is long. How much work is done in ploughing the length of the field?

** Answer: **

T he work done is given by :

or

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 :

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 :

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

What will be its kinetic energy when its velocity is increased three times?

** Answer: **

Using the formula for the kinetic energy we have :

or

or

** (i) Kinetic energy when velocity is doubled : **

** (ii) Kinetic energy when velocity is tripled : **

** 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 :

The SI unit of power is a watt.

** Q3. ** A lamp consumes of electrical energy in . What is its power?

** Answer: **

The power is given by :

Thus power is 100 watt.

** Answer: **

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

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

• 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.

** 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.

** Answer: **

By the equations of motion, we can write :

And the work done is :

Thus work done is - 210 J.

** Q5. ** A mass of is at a point on a table. It is moved to a point . If the line joining and 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 ** .

** 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.

** 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.

** Answer: **

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

Also,

Thus 250 units in joule is given by :

** Q 10. ** An object of mass _{} is raised to a height of 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 :

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

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

** 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.

** 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 Time

Thus the energy used in 10 hours is :

Hence the energy used by the heater is 15 KWh.

** 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, is moving with a constant velocity, . 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 :

Thus in bringing the object to rest the work needed is :

** 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.

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

** 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.

** 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 consumed in hours by four devices of power each.

** Answer: **

The energy consumed by 1 device is given by :

Thus the energy consumed by 4 devices is

** 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**

**NCERT Solutions for Class 9 - Subject Wise**

**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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