NCERT Solutions for Class 9 Science Chapter 10 Work and Energy

NCERT Solutions for Class 9 Science Chapter 10 Work and Energy help students understand the concepts of work, energy, and power. This chapter explains how energy is transformed from one form to another and introduces important laws like the law of conservation of energy. It also covers types of energy such as kinetic energy and potential energy.

Prepared according to the latest CBSE Class 9 Science syllabus, these solutions provide clear and structured answers to help students build strong concepts and perform well in exams.

NCERT Solutions for Class 9 Science Chapter 10 Work and Energy

Work and Energy
Medium
Q.
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.
Work and Energy
Easy
Q.
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.
Work and Energy
Medium
Q.
Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
Work and Energy
Easy
Q.
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?
Work and Energy
Medium
Q.
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.
 

Work and Energy
Difficult
Q.
Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Work and Energy
Difficult
Q.
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?
Work and Energy
Difficult
Q.
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?
Work and Energy
Medium
Q.
An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Work and Energy
Medium
Q.
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.
Work and Energy
Medium
Q.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Work and Energy
Easy
Q.
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 and Energy
Difficult
Q.
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.
Work and Energy
Medium
Q.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Work and Energy
Easy
Q.
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?
Work and Energy
Easy
Q.
What are the various energy transformations that occur when you are riding a bicycle?
Work and Energy
Medium
Q.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Work and Energy
Medium
Q.
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 and Energy
Medium
Q.
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.
Work and Energy
Medium
Q.
A battery lights a bulb. Describe the energy changes involved in the process.
Work and Energy
Medium
Q.
A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

NCERT Solutions for Class 9 Science Chapter 10 Work and Energy

Q1. Which of the following involve work?

Ans:

  • Swimming: Work is done (force causes displacement)
  • Donkey carrying load: No work (force ⟂ displacement)
  • Windmill lifting water: Work is done
  • Photosynthesis: No work (no displacement)
  • Engine pulling train: Work is done
  • Drying grains: No work
  • Sailboat moving: Work is done

Q2. Work done by gravity on a thrown object

Ans: Work done = 0 because initial and final heights are same, so vertical displacement is zero.

Q3. Work done by gravity when object moves horizontally

Ans: Work done = 0 since no vertical displacement.

Q4. Does decrease in potential energy violate conservation law?

Ans: No, because potential energy converts into kinetic energy, total energy remains constant.

Q5. Energy changes while riding a bicycle

Ans: Muscular energy → Kinetic energy + Heat energy.

Q6. Energy when pushing a rock that doesn’t move

Ans: Energy converts into heat energy, no work is done on rock.

Q7. Work done by gravity on satellite

Ans: Work done = 0 because force is perpendicular to displacement.

Q8. Can displacement occur without force?

Ans: Yes, if object moves with constant velocity (net force = 0).

Q9. Holding a load for long time

Ans: No work is done because no displacement occurs.

Q10. Pendulum and conservation of energy

Ans: Energy converts between kinetic and potential energy. Eventually stops due to air resistance, but total energy remains conserved.

Q11. Nature of work done

Ans:

  • Perpendicular force → Zero work
  • Same direction → Positive work
  • Opposite direction → Negative work

Q12. Can acceleration be zero with forces acting?

Ans: Yes, when forces balance each other, net force = 0.

Q13. What happens to kinetic energy after hitting ground?

Ans: It converts into heat, sound, and deformation energy.

FAQs – Class 9 Science Chapter 10 Work and Energy

Q1. What is work in physics?
Work is done when a force causes displacement in its direction.

Q2. What is energy?
Energy is the capacity to do work.

Q3. What are the types of energy?
Main types are kinetic energy and potential energy.

Q4. What is the law of conservation of energy?
Energy can neither be created nor destroyed, only transformed.

Q5. Why is this chapter important for exams?
This chapter includes numericals, concept-based, and reasoning questions, which are frequently asked in exams.