NCERT Solutions Class 9 Science Chapter 11

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

Science is a fascinating subject and also equally tough to comprehend. Students find it challenging to understand the concepts. Also, Class 9 Science is one of the most important of their academics, as it prepares them for the  higher  classes. Understanding the concepts and theories is essential for students who wish to make a career in Science and other technical fields. 

Class 9 Science Chapter 11: Work and Energy helps create a solid foundation for CBSE Term II exam preparation. The chapter introduces new concepts to the students. First, it talks about the essential ideas of ‘work’ and ‘energy.’ Students will learn about the conception of work as well as the influence of constant force on it. It will be followed by the concept of energy- ; students will learn to comprehend different forms of energy, such as potential and kinetic energy. Furthermore, students will learn the concepts of the potential energy of an object at a height and the interconversion of energy.

Extramarks NCERT Solutions Class 9 Science Chapter 11 are available for the students. Our subject matter experts  prepare the solutions diligently  and follow the latest CBSE guidelines. Needless to say, they completely understand what’s legitimate  asper the board’s standards 

NCERT Solutions consist of objective, diagram-based, short, and long-type questions. It can be helpful for both CBSE term-wise exams and other competitive exams. The solution guide helps students lay a good foundation for their CBSE term II preparation. Further, it consists of a comprehensive methodology  and a detailed step-by-step procedure.All the key points in each section are being written in the format required in CBSE examinations. These solutions will help you save time and pay attention to other subjects as well. That itself reduces stress and anxiety to a great extent. Thus, it proves to be a great guide for students to step up for their preparation to get excellent results. 

Extramarks is one of India’s leading online learning platforms, trusted by many students.   Students of primary and secondary schools  can sign up at Extramarks  website and explore other study material as per their requirement. The solutions are prepared  to help the students in every way. They need not look for any other assistance elsewhere Extramarks has solutions to all your problems.  

Students can visit our website for the latest updates and exam-related news. Further, students can also refer to NCERT solutions Class 10, NCERT solutions Class 11, and NCERT solutions Class 12. 

Key Topics Covered In NCERT Solutions for Class 9 Science Chapter 11:

NCERT Solutions Class 9 Science Chapter 11 elaborates new terms and definitions related to work and energy. Work as well as energy are closely related terms that we use daily. Here, students learn more about force, power, energy, and how they are interlinked. For the best learning experience, students can go through all the topics and sub-topics mentioned in NCERT solutions for Class 9 Science Chapter 11. 

Some of the key topics featured in NCERT solutions for Class 9 Science Chapter 11 are: 

1. Work

In scientific terms work is said to be done when a force is applied on an object and the object is getting displaced in the direction of the force applied. 

So we can define work as: 

“Work is the product of the force applied on an object in the direction of the displacement and the magnitude of this displacement.”

Work is a dot product of two vectors, force, and displacement. Work is, therefore, a scalar quantity. Hence, Joule (J) is the SI unit for work.

W = F.d

  • W = Work done
  • F = Magnitude of the force applied
  • d = displacement caused due to force

Students can refer to NCERT Solutions Class 9 Science Chapter 11 to understand the fundamental concepts of work and energy. 

2. Energy

Energy is the ability to heat or do work. It is a scalar quantity, meaning it has  magnitude but not direction. It is conserved, which means that energy can be changed from one form to the another , but it can neither be   created nor destroyed. There are different  types of energy: kinetic and potential energy; light energy , sound energy, nuclear energy, and light energy.

Energy is derived from the Greek term energeia, or the French words in and ergon, which both mean work. The SI unit for energy is the joule (1 J), where 1J = 1kgm2s-2. Other units include the kilowatt-hour (kW-hr), British thermal unit (BTU), and calorie (c). 1 J = 1kgm2s-2. 

Further, students can solve NCERT Solutions Class 9 Science Chapter 11 with more examples of    potential energy and kinetic energy. 

3. Kinetic Energy

Kinetic energy can be defined as energy produced by an object because of its motion. Specific forces are required to accelerate an object. It takes work to apply force. The energy is then transferred to the object, making it move at constant velocity. The energy that is transferred to the object is called kinetic energy. It depends on its speed and mass.

Kinetic Energy Examples elaborated in NCERT Solutions Class 9 Science Chapter 11: 

  • Students must bear in their mind the formula for kinetic energy and look at some examples from everyday life.
  • Because of its higher velocity and large mass, an aeroplane can fly with vast kinetic energy.
  • After being thrown, a baseball will have high kinetic energy due to its high velocity and small size.
  • Because of its high velocity and mass, a downhill skier will have immense kinetic energy when coming down from the top.
  • A golf ball struck before it has kinetic energy is zero.
  • Asteroids that fall at incredible speeds have a lot of kinetic energy.
  • Because of its smaller mass, a car moving down the road uses less energy than a semi-truck.

4. Potential Energy

The potential energy is in the form of energy that is stored in an object because of some position relative to another position at rest. Here are three types of energy effects: chemical, nuclear, and potential electric. It can be calculated based on the object’s mass, distance, and height. It is measured in Joules.

Rate of Doing Work

Machines do not consume or transfer energy at the same rate. Agents that transfer energy do work at different rates. Machines are classified based on how quickly they can change the energy or work. Power is given to agents who complete a task within a specified time.

Power = work/time

Students can refer to NCERT Solutions Class 9 Science Chapter 11 to comprehend the rate of doing work and understand  power. In addition,  Class 9 science  Chapter 11 question answers with detailed explanations are available for the students. 

NCERT Solutions for Class 9 Science Chapter 11: Exercise & Solutions

Students can refer to the NCERT solutions for Class 9 Science Chapter 11, which is available on our Extramarks’ website. These resources can help the students cover the entire syllabus and provide an in-depth analysis of the topics. The solution is now made easier for the students with the comprehensive language. Each case and subtopic is elaborated well for the students to understand and even to supplement their studies to avoid the last-minute hassle. Our solution is the one-stop guide to preparing for the exams. 

Chapter 11: Work and Energy  This chapter plays a vital role as it introduces students to concepts related to force, energy, potential energy, and the law of energy conservation. The chapter also has plenty of numerical solutions and to solve examples and solutions students may require help. They can refer to these NCERT solutions to get in-depth knowledge on essential topics of work and energy and verify their answers and stay on the course. 

Students can refer below for specific exercises,examples, and solutions:

  • Class 9 Science Chapter 11: Exercise 11.2 – Example and Solutions
  • Class 9 Science Chapter 11: Exercise 11.3 – Example and Solutions
  • Class 9 Science Chapter 11: Exercise 11.4 – Example and Solutions
  • Class 9 Science Chapter 11: Exercise 11.5 – Example and Solutions
  • Class 9 Science Chapter 11: Exercise 11.6 – Example and Solutions
  • Class 9 Science Chapter 11: Exercise 11.7 – Example and Solutions
  • Class 9 Science Chapter 11: Exercise 11.8 – Example and Solutions
  • Class 9 Science Chapter 11: Exercise 11.9 – Example and Solutions

Along with this, students can also refer to other solutions for primary and secondary classes:

  • NCERT Solutions Class 1
  • NCERT Solutions Class 2
  • NCERT Solutions Class 3
  • NCERT Solutions Class 4
  • NCERT Solutions Class 5
  • NCERT Solutions Class 6
  • NCERT Solutions Class 7
  • NCERT Solutions Class 8
  • NCERT Solutions Class 9
  • NCERT Solutions Class 10
  • NCERT Solutions Class 11
  • NCERT Solutions Class 12

NCERT Exemplar for Class 9 Science:

Science is fascinating and requires an in-depth understanding of the concepts. Answering a range of questions can help you to understand these concepts better. This helps students to clear their concepts  and gives them  a better learning environment . In addition, solving different questions with a higher difficulty level will help students gain confidence in remembering the concepts. 

NCERT Exemplars help students to improve their understanding of issues.  There are numerous questions with varying  degrees of difficulty level.  In Chapter 11, there are  examples and solutions followed by chapter end exercises. . Students will get to practise  miscellaneous questions at the end of the exemplar. It’s better to go through the NCERT book first then try exemplar to maximise your potential.

The exemplar also has a few questions from NCERT Solutions Class 9 Science Chapter 11. Students will be able to examine different sub-topics, such as the law of conservation of energy, potential energy, kinetic energy and rate of doing work.

Key Features of NCERT Solutions for Class 9 Science Chapter 11:

Students are encouraged to study NCERT solutions and textbooks to prepare for the Class 9 Science annual exams. NCERT Solutions for Class 9 Science chapter 11 helps students to answer different types of questions and  to enhance their exam preparation so that they can face any kind of challenging question with ease and be confident.. These solutions use a simple language and a step-by, systematic approach to help students understand scientific concepts. Students can also refer to the chapter-wise NCERT Solutions to CBSE Class 9 Science to supplement their learning.

The key concepts from NCERT Solutions Class 9 Science Chapter 11 are listed here: 

  • It will help students revise concepts with examples provided, so they can easily understand them. 
  • The solution illustrates using scientific ideas in real life situations like designing a car, designing a power station, etc. to  clarify their concepts.
  • Our solution also provides essential information and background material relevant to the chapter. This will help students understand and remember significant points from the chapter easily. 
  • The notes are prepared by highly qualified and experienced faculty who meticulously follow the NCERT textbooks and CBSE guidelines to provide authentic and reliable study material.
  • The solutions are provided in an elementary language so as not to overwhelm students, but at the same time, they cover all the necessary details needed by students.
  • The explanations and examples of NCERT Solutions for Class 9 Science chapter 11 have been kept simple and easy to understand. The solution is arranged step-by-step, with each model and explanation marked by its questions.
  •  It’s important to go beyond the NCERT books by including solutions as an integral  part of your  study schedule. This encourages the students to master the topic and increases their confidence in achieving a high grade. Students swear by Extramarks because of the absolute trust and confidence it has built over the years. 

Q.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’.

• 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.


• While swimming, Suma exerts a force on the water in backward direction. Hence, she swims in the forward direction caused by the forward reaction of water. Thus, the force causes a displacement. Hence, Suma does the work while swimming.

• When the donkey carries a load, it applies a force in the upward direction while displacement of the load is in the forward direction. In this case, displacement is perpendicular to the force. Hence, the work done is zero.

• A wind mill pulls water against the gravitational force. Thus, work is done by the wind mill to lift the water from the well.

• In photosynthesis process, there is no displacement of the leaves of the plant. Hence, the work done is zero.

• A force is applied by engine on the train. Therefore, the train moves in the direction of applied force. Here, displacement is in the direction of force applied. Hence, work is done by the engine on the train.

• 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 presence of the Sun.

• Wind exerts a force on the sailboat to push it in the forward direction. Thus, there is a displacement in the boat in the direction of force. Hence, work is done by wind on the boat.

Q.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?


Work done by the force of gravity on an object depends only on vertical displacement. Vertical displacement can be calculated by the difference in the initial and final heights of the object. Here, vertical displacement is zero. Hence, work done by gravity W = mgh

Here, h = 0

Therefore, W = mg × 0 = 0

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


The chemical energy of battery is changed into electrical energy when a bulb is connected between the terminals of a battery. Later on, this electrical energy changes into light and heat energy. Hence, the transformation of energy in the given situation can be shown as:

ChemicalenergyElectricalEnergyLightEnergy+HeatEnergy MathType@MTEF@5@5@+=feaaguart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0le9yqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeaacaGaaiaabeqaamaabaabaaGcbaGaae4qaiaabIgacaqGLbGaaeyBaiaabMgacaqGJbGaaeyyaiaabYgacaaMe8Uaaeyzaiaab6gacaqGLbGaaeOCaiaabEgacaqG5bGaaGjbVlabgkziUkaaysW7caqGfbGaaeiBaiaabwgacaqGJbGaaeiDaiaabkhacaqGPbGaae4yaiaabggacaqGSbGaaGjbVlaabweacaqGUbGaaeyzaiaabkhacaqGNbGaaeyEaiaaysW7cqGHsgIRcaaMe8UaaeitaiaabMgacaqGNbGaaeiAaiaabshacaaMe8Uaaeyraiaab6gacaqGLbGaaeOCaiaabEgacaqG5bGaaGjbVlaabUcacaaMe8UaaeisaiaabwgacaqGHbGaaeiDaiaaysW7caqGfbGaaeOBaiaabwgacaqGYbGaae4zaiaabMhaaaa@7861@

Q.4 Certain force acting on 20 kg mass changes its velocity from 5 ms−1 to 2 ms−1. Calculate the work done by the force.


Kinetic energy E = 12mv2Where, mass, m = 20 kg, v = velocity(i) Kinetic energy E1, when velocity is 5 ms-1 K1 = 12(20kg)(5ms1)2=250 J(ii) Kinetic energy (K2) when velocity is 2 ms-1 K2=12(20kg)(2ms1)2= 40 JWork done is equal to the change in the kinetic energyHence, W = K2K1 =40250= -210 J

Q.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.


Work done by gravity depends only on the vertical displacement of the body. The expression for work done by gravity is given by,

W = mgh

Here, vertical displacement (h) is zero.

Hence, W = mg × 0 = 0

Thus, work done by gravity on the body is zero.

Q.6 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 freely from a height, then its potential energy converts into kinetic energy with time. The decrease in the potential energy is equal to the increase in the kinetic energy of the body. Thus, total mechanical energy of the body remains conserved.

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


In the case of bicycle riding, the muscular energy of the rider changes into heat energy and kinetic energy of the bicycle. Heat energy gives heat to the rider’s body while kinetic energy provides a velocity to the bicycle. In this whole process, the total energy remains conserved.

Q.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?


On pushing a huge rock, no muscular energy is transferred to the stationary rock. Here, muscular energy is transferred into heat energy which makes the body feel warm. Hence, there is no loss of energy.

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


Here, 1unit=1kWh Also, 1kWh=3.6×1 0 6 J 250units ofenergy=250 ×3.6×1 0 6 J= 9×1 0 8 J MathType@MTEF@5@5@+=feaaguart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqipz0xg9vqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=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@8C0D@

Q.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.


Given, Height of object, h = 5 m mass of object, m = 40 kg acceleration due to gravity, g = 9 .8 ms -2 Gravitational potential energy is given by, W = mgh or, = 40 kg × 9 .8 ms -2 × 5 m= 1960 J Now, potential energy at half-way = 1960 J 2 = 980J. At this point, the object has an equal amount of potential energy and kinetic energy. Thus, half – way down, Kinetic energy = 980J.

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


There are two conditions when the work is said to be done.

(i) A force acts on the body.

(ii) There is a displacement of the body due applied force in or opposite to the direction of force.

If the direction of force is perpendicular to displacement, then the work done is zero.

When a satellite revolves around the Earth, then the direction of force of gravity on the satellite is perpendicular to its displacement. Thus, the work done on the satellite by the Earth is zero.

Q.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. When an object is moving with constant velocity, the net force acting on it is zero. However, there is a displacement along the motion of the object. Therefore, there can be a displacement of an object without any force acting upon it.

Q.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.


When the person holds a bundle of hay over his head, then bundle of hay is not displaced. However, force of gravity is still working on the bundle but the person is not applying any force on it. Thus, work done by the person on the bundle is zero.

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


Given, Power, P = 1500 W = 1.5 kW Time, t = 10 h Now, Work done = energy consumed by the heater and Energy consumed = Power × time Hence, = 1.5 kW × 10 h = 15 kWh Thus, energy consumed by the heater in 10 h is 15 kWh. MathType@MTEF@5@5@+=feaaguart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVv0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=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@1A74@

Q.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?


According to law of conservation of energy, energy can be neither created nor destroyed. It can only be converted from one form to another.

When a pendulum oscillates from its mean position O to either of its extreme positions P or Q, it rises through a height h above the mean level P. At this point, the kinetic energy of the bob changes completely into potential energy. The kinetic energy becomes zero, and the bob possesses only potential energy. As it goes towards point O, its potential energy decreases progressively. Accordingly, the kinetic energy increases. As the bob reaches point O, its potential energy becomes zero and the bob possesses only kinetic energy. This process is repeated as long as the pendulum oscillates.

After some time, bob comes to rest because air resists its motion. The pendulum loses its kinetic energy to overcome this friction and stops after some time.

Here, the law of conservation of energy is not violated because the energy lost by the pendulum to overcome friction is gained by its surroundings. Thus, the total energy of the pendulum and the surrounding system remain conserved.

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?


When an object of mass ( m ) is moving with a velocity ( v ) then, its kinetic energy ( K ) is given as, K = 1 2 mv 2 In order to bring the object to rest, an amount of work is need to be done that which is equal to the kinetic energy of the object. Hence, work done ( W ) on the object to bring it to rest would be, W = 1 2 mv 2 .

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


Given, Mass, m = 1500 kg Velocity, v = 60 kmh -1 = 60 × 5 18 ms -1 The work done to stop the car would be equal to the kinetic energy acquired by it. Hence, kinetic energy, E k = 1 2 mv 2 E k = 1 2 ×1500× ( 60× 5 18 ) 2 =208333.3 J Hence, 20 8333.3 J of work is required to stop the car.

Q.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.

1. In first case, the direction of force acting on the block is perpendicular to the displacement. Thus, work done by force on the block will be zero.

2. In the second case, 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.

3. In the third case, 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.

Q.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?


The statement of Soni is correct. Acceleration in an object may be zero even when several forces are acting on it. This condition occurs when all the forces cancel out each other. For a uniformly moving object, the net force acting on the object is zero. Hence, the acceleration of the object is zero.

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


Given, Power of each device, P = 500 W = 0.50 kW Time, t = 10 h Energy consumed = Power × Time = 0.50 kW × 10 h = 5 kWh Hence, the energy consumed by four equal rating devices in 10 h = 4 × 5 kWh = 20 units.

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


In free fall, potential energy of an object decreases and kinetic energy increases. When the object comes in contact with ground, potential energy converts into kinetic energy. As the object hits the hard ground, all its kinetic energy gets converted into heat energy and sound energy. The object can also deform the ground depending upon the nature of the ground and the amount of kinetic energy possessed by the object.

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FAQs (Frequently Asked Questions)

1. Why should we rely on Extramarks NCERT solutions?

NCERT solutions for class 9 Science Chapter 11 are designed to give students a clear idea of what they should be focusing on when preparing for exams. You will access multiple question banks and summaries of each topic and subtopic. All questions, answers, and explanations are clear, concise, understandable, and reliable to help you prepare for exams. The suitable study materials will allow you to gain the knowledge and understanding necessary for work and energy.

2. What is the kinetic energy of an object?

Kinetic energy is the amount of energy an object has due to its motion. It’s the work needed to propel a mass from its rest to its actual velocity. So kinetic energy is the energy required to move an object. Kinetic Energy can be seen in a person running or walking, throwing a baseball, and a crumb falling off a tabletop. Visit the Extramarks website or mobile app to learn more about the chapter.

3. How to benefit from NCERT solutions for Class 9 Science Chapter 11?

Students can refer to our NCERT solutions for Class 9 Science chapter 11. It offers comprehensive and detailed answers to every type of question present. Students can practise with the solutions and understand the concepts with detailed  explanations. It also provides a plethora of  exercise questions which help students to prepare for their exams. It is available in  simple and easy  language. Above all, these solutions come handy especially during last minute preparation and saves time for further revision.