NCERT Class 9 Science Chapter 10 ‘Work and Energy’ Questions and Answers

Contents

Introduction:

Welcome to our online tutorial classes, where learning meets innovation! In this segment, we embark on an exciting journey into the realm of matter with our meticulously crafted NCERT Class 9 Science Chapter 10 questions and answers on “Work and Energy.” Through these questions and answers, we aim to ignite your curiosity, deepen your understanding, and empower you with knowledge that transcends the boundaries of the classroom.

NCERT Class 9 Science Chapter 10, ‘Work and Energy’, explores the concepts of work, energy, and power. It discusses kinetic and potential energy, the law of conservation of energy, and the conversion of energy forms. The chapter includes practical examples and activities for better understanding.

NCERT Class 9 Science book:

Click here to access and download the NCERT class 9 Science book.

NCERT class 9 Science Chapter 10 ‘Work and Energy’ Overview

NCERT Class 9 Science Chapter 10, ‘Work and Energy’, delves into the fundamental concepts of work, energy, and power. It explains how work is a measure of energy transfer and introduces different forms of energy, including kinetic and potential energy. The chapter emphasizes the law of conservation of energy and the interconversion of energy forms. It also discusses the concept of power as the rate of doing work or transferring energy. Practical activities and real-life examples are included to enhance understanding of these concepts and their applications in daily life.

With our expertly curated notes, you’ll be well-equipped to ace your CBSE Class 9 Science exams and embark on a lifelong journey of discovery and learning.

NCERT class 9 Science Chapter 10 ‘Work and Energy’ Questions and Answers

Part 1

Question 1

The displacement is, say 8 m, in the direction of the force (Fig. 10.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case?

Solution:

Displacement = 8 m

Force = 7 N

$$ Work \; done = Force \times displacement $$ $$ W = 7 \; N \times 8 \; m = 56 \; Nm = 56 J $$

Hence, the work done in this case is 56 Joules.

Part II

Question 1

When do we say that work is done?

Answer:

  • Work is said to be done when a force acts on an object and the object is displaced in the direction of the force.
  • If either the force or the displacement is zero, or if the force is perpendicular to the direction of displacement, then no work is done.

Question 2

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

Solution:

The expression for work done (W) when a force (F) is acting on an object in the direction of its displacement (s) is given by:

$$ W = F \times s $$

Question 3

Define 1 J of work.

Answer:

One joule (1 J) of work is defined as the amount of work done when a force of one newton (1 N) causes a displacement of one meter (1 m) in the direction of the force. In other words, 1 J = 1 Nm.

Question 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?

Solution:

Force = 140 N

Length of the field = 15 m

Therefore

$$ Word \; done = F \times s = 140 \; N \times 15 \; m = 2100 \; Nm = 2100 \; J $$

Hence, the work done in ploughing the length of the field is 2100 Joules.

Part III

Question 1

What is the kinetic energy of an object?

Answer:

The kinetic energy of an object is the energy that it possesses due to its motion. It is dependent on both the mass and the speed of the object.

Question 2

Write an expression for the kinetic energy of an object.

Answer:

The expression for the kinetic energy (KE) of an object is given by the formula:

$$ E_k = \frac{1}{2} mv^2 $$

Where:

KE = Kinetic Energy (in joules)

m = Mass of the object (in kilograms)

v = Velocity of the object (in meters per second)

Question 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?

Solution:

velocity of the object = 5 m/s

Kinetic Energy of the object = 25 J

Therefore, mass ‘m’ of the object $$ = 2 \times \frac{E_k}{v^2} = 2 \times \frac{25 \; J}{(5ms^{-1})^2} = 2 \; kg $$

(i) Kinetic Energy when the velocity is doubled v = 2 x 5 = 10 m/s

$$ E_{k1} = \frac{1}{2} \times m v^2 = \frac{1}{2} \times 2 \times 10^2 = 100 \; J $$

(ii) When the velocity is increased three time v = 3 x 5 = 15 m/s

$$ E_{k2} = \frac{1}{2} \times 2 \times 15^2 = 225 \; J $$

Therefore, when the velocity is doubled, the kinetic energy becomes 100 J, and when the velocity is increased three times, the kinetic energy becomes 225 J.

Part IV

Question 1

What is power?

Answer:

  • Power is defined as the rate of doing work or the rate of transfer of energy.
  • If an agent does a work W in time t, then power is given by:
$$ Power = \frac{work}{time} $$ $$ P = \frac{W}{t} $$
  • The unit of power is watt (W),

Question 2

Define 1 watt of power.

Answer:

One watt of power is defined as the rate of work done or energy transfer equivalent to one joule per second. In other words, if one joule of work is done in one second, the power involved in the process is one watt.

Question 3

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

Energy consumed = 1000 J

Time taken = 10 s

Power is defind as $$ = \frac{Energy}{Time} = \frac{1000}{10} = 100 \; W $$

Therefore, the lamp has a power consumption of 100 watts.

Question 4

Define average power.

Answer:

Average power is the total energy consumed or work done divided by the total time taken. It gives the average rate at which work is done or energy is transferred over a period of time. Mathematically, average power is given by:

$$ P_{avg} = \frac{W_{total}}{t_{total}} $$ $$ where \; W_{total} \; \text{is total work done or energy consumed} $$ $$ and \; t_{total} \; \text{is total time taken} $$

Exercises

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’.

  • 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:

work is done when a force causes displacement in the direction of the force.

  • Suma is swimming in a pond: Work is being done as Suma is applying force to move through the water.
  • A donkey is carrying a load on its back: If the donkey is moving, then work is being done as the donkey is applying force to move the load.
  • A wind-mill is lifting water from a well: Work is being done as the windmill applies force to lift water against gravity.
  • A green plant is carrying out photosynthesis: No physical work is done in the physics sense as there’s no mechanical force causing a displacement. However, there’s chemical work being done at the molecular level.
  • An engine is pulling a train: Work is being done as the engine applies force to move the train.
  • Food grains are getting dried in the sun: No work is done in the physics sense as there’s no force causing a displacement.
  • A sailboat is moving due to wind energy: Work is being done as the wind applies force to move the sailboat.

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?

Answer:

The force of gravity acts vertically downwards. In this scenario, the object’s entire path is horizontal. Since there’s no displacement in the direction of the force (gravity), no work is done by gravity on the object.

Question 3

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

Solution:

In the process of a battery lighting a bulb, the energy changes involved are as follows:

  • The chemical energy stored in the battery is converted into electrical energy as the battery provides the necessary voltage to the circuit.
  • The electrical energy flows through the circuit and reaches the bulb, where it is converted into light energy and thermal energy (heat) as the bulb filament heats up and emits light.
$$ Chemical \; energy \rightarrow Electrical \; energy \rightarrow Light \; energy \rightarrow Heat \; energy $$

Question 4

Certain force acting on a 20 kg mass changes its velocity from 5 m s–1 to 2 m s–1. Calculate the work done by the force.

Solution:

Mass of the body = 20 kg

change in velocity = 5 – 2 = 3 m/s

Work done

$$ = \frac{1}{2} \times m (v – u)^2 = \frac{1}{2} \times 20 \times (2^2 – 5^2) = -210 \; J $$

The negative sign indicates that the work is done against the direction of motion.

Question 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.

Solution:

If the mass is moved horizontally on the table, the work done by the gravitational force is zero. This is because the gravitational force acts vertically downward, while the displacement of the mass is horizontal. Since the angle between the force and the displacement is 90 degrees, so the work done is zero.

Question 6

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

Answer:

The decrease in potential energy of a freely falling object does not violate the law of conservation of energy. According to the law of conservation of energy, energy can neither be created nor destroyed, but it can be transformed from one form to another. In the case of a freely falling object, the decrease in potential energy is accompanied by an increase in kinetic energy, as the object gains speed while falling. Therefore, the total mechanical energy (sum of potential energy and kinetic energy) of the object remains constant throughout its fall, thus obeying the law of conservation of energy.

Question 7

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

Answer:

Energy Transformations while Riding a Bicycle:

  • Muscular energy from the cyclist is converted into mechanical energy to pedal the bicycle.
  • Mechanical energy is transferred to the bicycle’s wheels, causing them to rotate.
  • The rotational motion of the wheels generates kinetic energy, propelling the bicycle forward.
  • Frictional forces between the tires and the road surface dissipate some of the energy as heat.

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?

Answer:

Yes, energy transfer occurs even if the rock doesn’t move.

  • Muscles use Muscular energy to contract and exert a force.
  • The force applied is transferred to the ground through feet (mechanical energy transfer).
  • Since the rock doesn’t move, the force isn’t enough to overcome its inertia.
  • The energy expent is mainly converted into heat due to muscular contractions and friction between feet and the ground.

Question 9

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

Answer:

1 unit of energy consumption equals 1 kWh (kilowatt-hour). We can convert this to joules:

  • 1 kWh = 1 kilowatt x 1 hour
  • 1 kWh = (1000 watts) x (3600 seconds)
  • 1 kWh = 3,600,000 J

Therefore, the household consumed 250 units x 3,600,000 J/unit = 900,000,000 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.

Answer:

Mass of the object = 40 kg

Height to which the object is raised = 5 m

Therefore potential energy gained

$$ E_p = mgh = 40 \; kg \times 10 ms^{-2} \times 5 \; m = 2000 \; J $$

Height of the object when its midway = 2.5 m

Therefore the potential energy at 2.5 m height

$$ E_{p’} = mgh = 40 \; kg \times 10 \; ms^{-2} \times 2.5 \; m = 1000 \; J $$

Therefore Kinetic energy will be

$$ 2000 \; J – 1000 \; J = 1000 \; J $$

Question 11

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 force of gravity on a satellite moving round the Earth is zero. This is because the force of gravity acts towards the center of the Earth, while the displacement of the satellite is perpendicular to this force (along the circular orbit) and the angle between the force and displacement is 90 degrees, so the work done 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.

Answer:

Yes, there can be displacement of an object in the absence of any net force acting on it. According to Newton’s first law of motion, an object in motion continues to stay in motion with the same speed and in the same direction, unless acted upon by an unbalanced force. So, if an object is already moving and no net force is acting on it, it will continue to move and hence, be displaced.

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.

Answer:

We know that:

$$ Work \; done = Force \times Displacement $$

Here since the displacement is zero, hence the work done is zero.

Question 14

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

Solution:

We know that

$$ Energy = Power \times time = 1500 W \times 10 \; h = 15000 \; wh = 15 \; kwh $$

Hence, the heater uses 15 kwh of energy in 10 hours.

Question 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?

Answer:

  • Initial: The bob has maximum potential energy (PE) when pulled to one side (highest point) and minimal kinetic energy (KE) (at rest).
  • Release: As you release the bob, PE starts decreasing, and KE increases due to gravity pulling it down (conversion of PE to KE).
  • Swinging: At the bottom of the swing, the bob has minimal PE (lowest point) and maximum KE (highest speed).
  • Energy Transfer: Due to air resistance and friction at the pivot point, a small amount of mechanical energy is continually lost and converted into heat.
  • Gradual Stop: Over time, due to energy loss, the bob’s swings become smaller (decreasing KE). Eventually, it comes to rest at the center point where both PE and KE are minimal.

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?

Answer:

Mass of the object = m

Velocity = v

Therefore Kinetic energy possessed by the body

$$ E_k = \frac{1}{2} \times mv^2 $$

Therefore, the work done on the object to bring it to rest is equal to is 1/2 mv².

Question 17

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

Answer:

Mass of the car = 1500 kg

Velocity = 60 km/h = 16.67 m/s

Work required to be done

$$ = \frac{1}{2} \times 1500 kg \times (16.67)^2 = 208416.68 \; J = 2084.17 \; KJ $$

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:

Case I

Zero Work: The force (F) is perpendicular to the direction of displacement, hence the work done is zero.

Case II

Positive Work: The direction of force (F) is along with the direction of displacement (west to east), the work done is positive.

Case III

Negative Work: The direction of force (F) is opposite to the direction of displacement, the work done is negative (work done against the displacement).

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?

Answer:

Yes, I agree with Soni. The acceleration of an object could be zero even when several forces are acting on it. This happens when the vector sum (or resultant) of all the forces acting on the object is zero. This condition is known as equilibrium.

Question 20

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

Solution:

Power of each device = 500 W

Therefore total power = 500 x 4 = 2000 W

Time duration = 10 hours

Therefore

$$ Energy consumed = Power \times time $$ $$ = 2000 \; W \times 10 \; h = 20000 \; W = 20 \; kWh $$
$$ = 20 \times 10 \times 60 \times 60 = 72000 \; KJ $$

Question 21

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

Solution:

Under the free fall, the potential energy of the object is converted to Kinetic energy. As the object strikes the ground, all its potential energy is converted to kinetic energy.

When a freely falling object eventually stops on reaching the ground, its kinetic energy is converted into other forms of energy. As the object comes to rest, its kinetic energy is dissipated into the surroundings, typically through vibrations and heat generated upon impact with the ground. Some of it goes into deforming the object or the ground (depending on the hardness of the ground and the object). This does not violate the law of conservation of energy, as the total energy (including kinetic, potential, heat, and sound) remains constant.

Engage and Excel

Our NCERT Class 9 Science Chapter 10 ‘Work and Energy’ questions and answers offer a dynamic and interactive learning experience. Dive into engaging visuals, and interactive quizzes that bring the concepts of matter to life. With our user-friendly interface and comprehensive content, learning becomes not just effective but also enjoyable.

Join us on this captivating journey as we unravel the mysteries of matter in our surroundings. With our expertly curated notes, you’ll be well-equipped to ace your NCERT Class 9 Science exams and embark on a lifelong journey of discovery and learning.

Let’s Begin Your Learning Journey:

Join our online tutorial classes and embark on an enriching educational journey with us! At Online Tutorial Classes, we strive to provide students with comprehensive resources and expert guidance to excel in their academic pursuits. Whether you’re seeking support for NCERT Class 9 Science or other subjects, our interactive tutorials, engaging content, and personalized coaching are designed to cater to your learning needs. Connect with us today to explore our offerings further and discover how we can help you achieve your academic goals. Let’s embark on this learning adventure together!

Contact Us

For further details and inquiries, don’t hesitate to contact us here. Let’s embark on this educational journey together and make learning an inspiring adventure!

Advantages of Choosing Our Coaching Classes

  1. Structured Curriculum: Our coaching classes follow a structured curriculum aligned with the CBSE guidelines, ensuring comprehensive coverage of all topics.
  2. Experienced Educators: Learn from experienced educators who are experts in their respective fields. Our faculty members are dedicated to providing high-quality instruction and support to help students succeed.
  3. Personalized Attention: Receive personalized attention and support from our instructors. We believe in nurturing each student’s unique talents and addressing their individual learning needs.
  4. Regular Assessments and Feedback: Benefit from regular assessments and feedback to track your progress and identify areas for improvement. Our comprehensive evaluation system ensures that you stay on track towards achieving your academic goals.
  5. Holistic Development: Beyond academic excellence, our coaching classes focus on fostering holistic development. We aim to equip students with the skills, knowledge, and confidence needed to thrive in all aspects of life.

Our Commitment:

At Online Tutorial Classes, we are committed to supporting your educational journey. We believe that providing easily accessible resources can make a significant difference in your academic success.

Accessing All Chapters of NCERT Class 9 Science:

If you’d like to explore more chapters from the NCERT Class 9 Science book, you can easily access them on our website. We’ve organized all the chapters in one convenient location, making it effortless for you to navigate and study at your own pace. Simply click on the link below to access the page containing links to all the chapters of the book.

CBSE class 9 Science Notes

We believe that a holistic understanding of the entire curriculum is essential for your academic growth, and our platform is designed to support you in achieving just that.

Accessing the Questions and Answers:

  1. Select Your Class: To get started, choose Class 9 from the drop-down menu on our website.
  2. Choose Your Subject: Select “Science” to access the relevant material.
  3. Navigate to the Chapter: Locate and click on the chapter
  4. Access Questions and Answers: You will find a list of questions related to this chapter. Click on any question to view its answer along with a detailed explanation.

Thank you for choosing Online Tutorial Classes as your trusted source for chapter-wise NCERT Class 9 Science Chapter 10 ‘Work and Energy’ questions and answers. Let’s embark on a journey of academic excellence together.

Related Topics

Leave a Reply

© 2026 - All rights reserved
Go to top

Discover more from

Subscribe now to keep reading and get access to the full archive.

Continue reading