PhysicsICSE Class 8 · Chapter 4

Energy ICSE Class 8 Physics: Work, Kinetic and Potential Energy, and Power

Updated 15 October 2026 · Fact-checked

Work is done when a force moves a body: W = F × s, measured in joules. Energy is the capacity to do work. In ICSE Class 8, kinetic energy is ½mv², gravitational potential energy is mgh (with g = 10 m/s²), energy changes from one form to another, and power is the rate of doing work, in watts.

What Energy is about

In physics, work has a special meaning: work is done only when a force moves a body in the direction of the force. Energy is the capacity (ability) to do work, so work and energy are measured in the same unit, the joule (J).

The CISCE Class VIII curriculum asks you to:

understand the concept of work and its unit, and calculate work in simple cases,
understand kinetic energy (energy of motion) and potential energy (stored energy), especially gravitational potential energy,
calculate kinetic and potential energy from given data (simple problems, taking $g = 10 m/s^{2}$ ),
describe energy transformations in daily life,
explain the difference between energy and power.

Useful units: $1 kJ = 1000 J$ ; power in watt (W), where $1 W = 1 J/s$ .

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Physics: your school sets and marks the exam. Energy sits in the Theme 4: Energy theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Work

Work is done when a force acts on a body and the body moves in the direction of the force:
$W = F \times s$ ,
where $F$ is the force (N) and $s$ the displacement (m) in the direction of the force.

Unit: joule (J). 1 joule of work is done when a force of 1 N moves a body through 1 m in its direction.

No work is done if:

there is no displacement (pushing a wall that does not move), or
the motion is at right angles to the force (in physics, holding a bag still or carrying it on a level path does no work against gravity).

Worked example

A girl pushes a box with a force of 50 N, and it moves 4 m in the direction of the push. How much work does she do?

Try it first, then show the solutionSolution

$W = F \times s = 50 \times 4$ .
$= 200$ J.

Answer: 200 J

Work done against gravity

To lift a body of mass $m$ steadily, you must apply a force equal to its weight, $m g$ . If it rises through a height $h$ :
$W = m g \times h$ .

Take $g = 10 m/s^{2}$ , so a mass of 1 kg weighs 10 N.

This work is stored in the body as gravitational potential energy.

Worked example

How much work is done in lifting a 5 kg bag to a shelf 2 m high? ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

Weight $= m g = 5 \times 10 = 50$ N.
$W = 50 \times 2 = 100$ J.

Answer: 100 J

Kinetic energy

Kinetic energy (KE) is the energy a body has because it is moving:
$KE = \frac{1}{2} m v^{2}$ ,
with $m$ in kg and $v$ in m/s, giving KE in joules.

Double the mass → KE doubles.
Double the speed → KE becomes four times as large (because $v$ is squared).

Examples: a moving car, flowing water, wind, a rolling ball, a hammer swinging down.

Worked example

Find the kinetic energy of a 2 kg ball moving at 3 m/s. What is its KE at 6 m/s?

Try it first, then show the solutionSolution

$KE = \frac{1}{2} \times 2 \times 3^{2} = 9$ J.
At 6 m/s: $\frac{1}{2} \times 2 \times 36 = 36$ J, which is 4 times as much.

Answer: 9 J; 36 J

Potential energy

Potential energy (PE) is energy stored in a body because of its position or shape.

Gravitational PE: energy of a body raised above the ground: $PE = m g h$ .
Energy stored by changing shape: a stretched rubber band, a wound spring, a bent bow.

The higher a body is raised, or the heavier it is, the more PE it has. Water stored high up in a dam has a large PE.

Worked example

Find the potential energy of a 2 kg flower pot on a balcony 5 m above the ground. ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

$PE = m g h = 2 \times 10 \times 5$ .
$= 100$ J.

Answer: 100 J

Energy transformations

Energy cannot be made or destroyed; it changes from one form to another.

Situation	Energy change
Falling ball	potential → kinetic
Torch	chemical (cell) → electrical → light (and heat)
Electric fan	electrical → kinetic (and some sound, heat)
Hydroelectric power station	potential (water) → kinetic → electrical
Burning fuel in a car	chemical → heat → kinetic
Solar cell	light → electrical

For a body falling freely (ignoring air), the PE lost equals the KE gained.

Worked example

A 2 kg ball falls from rest from a height of 5 m. Ignoring air resistance, find its kinetic energy and speed just before it hits the ground. ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

PE at the top $= 2 \times 10 \times 5 = 100$ J; all of it becomes KE: $KE = 100$ J.
$\frac{1}{2} \times 2 \times v^{2} = 100$ , so $v^{2} = 100$ and $v = 10$ m/s.

Answer: 100 J; 10 m/s

Energy and power

Energy is the capacity to do work (how much work can be done). Power is the rate of doing work (how fast it is done):
$P = \frac{W}{t}$ .

Unit of power: watt (W); $1 W = 1 J/s$ ; $1 kW = 1000$ W.

Energy	Power
capacity to do work	rate of doing work
unit: joule	unit: watt
does not depend on time taken	depends on time taken

Two people doing the same work have used the same energy, but the one who does it faster is more powerful.

Worked example

A boy weighing 500 N runs up stairs 6 m high in 12 s. Find the work done and his power.

Try it first, then show the solutionSolution

$W = 500 \times 6 = 3000$ J.
$P = \frac{3000}{12} = 250$ W.

Answer: 3000 J; 250 W

Physics in real life: Energy

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Work against gravity

Suppose you lift your 200 g phone from the floor onto a shelf 1.5 m high.

How much work do you do on it? ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

$m = 0.2$ kg; weight $= 0.2 \times 10 = 2$ N.
$W = 2 \times 1.5 = 3$ J.

Answer: 3 J

Memory hook: Lifting work = weight × height.

Power and energy

Imagine a 60 W bulb left on for one hour.

How much energy does it use?

Try it first, then show the solutionSolution

1 hour $= 3600$ s; energy $= P \times t$ .
$60 \times 3600 = 216000$ J $= 216$ kJ.

Answer: 216 000 J

Memory hook: Energy = power × time.

Kinetic energy

Suppose you and your bicycle have a total mass of 60 kg and you ride at 5 m/s.

What is your kinetic energy?

Try it first, then show the solutionSolution

$KE = \frac{1}{2} \times 60 \times 5^{2}$ .
$= 30 \times 25 = 750$ J.

Answer: 750 J

Memory hook: Faster riding stores much more KE, since speed is squared.

Topics in this chapter

Energy formula sheet

Work: $W = F \times s$; Joule (J).
Kinetic energy: $KE = \frac{1}{2} m v^{2}$; m in kg, v in m/s.
Potential energy: $PE = m g h$; g = 10 m/s².
Power: $P = \frac{W}{t}$; Watt (W) = J/s.
Units: $1 kJ = 1000 J$; 1 kW = 1000 W.

Energy: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Energy changes of a falling ball

Show the energy changes as a ball falls from a height.

Draw

A ball at the top of a vertical dotted path, the same ball half-way down, and at the bottom just above the ground.
A small bar beside each position showing PE and KE.

Label

Top: maximum PE, zero KE
Half-way: PE = KE
Bottom: zero PE, maximum KE
Height h

Watch out: State that air resistance is ignored, so the total (PE + KE) stays the same.

How the school exam asks Energy

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Work done	Find the work done by a force or in lifting a mass.	W = F × s; for lifting, F = mg.
Kinetic energy	Find KE; compare KE when speed or mass is doubled.	½mv²; speed is squared.
Potential energy	Find PE of a body at a height; find the height from PE.	mgh with g = 10; h = PE ÷ (mg).
Energy changes	State the energy changes in a torch, a fan, a falling ball.	List the forms in order.
Power	Find the power of a person or machine; distinguish energy and power.	P = W/t; same work faster means more power.

Tricks to remember Energy

Short memory aids. Say each one aloud once, then use it on a question the same day.

Half m v squared
Say it as 'half-em-vee-squared': KE = ½mv².
Use it when: Remembering KE.
m-g-h: mass, gravity, height
PE = mgh, the three things that make a raised object dangerous.
Use it when: Remembering PE.
No move, no work
No displacement in the direction of the force means zero work, however tired you feel.
Use it when: Work = 0 cases.
Power = speed of work
Energy says how much; power says how fast.
Use it when: Energy versus power.

Energy: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Unit of work · 1 mark · MCQExam level

The SI unit of work is:
(a) newton
(b) watt
(c) joule
(d) pascal

Try it first, then show the solutionSolution

Work = force × displacement: N × m.
1 N m of work is 1 joule.

Answer: (c)

Example 2: Simple work · 2 marks · Short answerExam level

A force of 30 N moves a trolley 5 m in its direction. Find the work done.

Try it first, then show the solutionSolution

$W = F \times s = 30 \times 5$ .
$= 150$ J.

Answer: 150 J

Example 3: KE of a car · 3 marks · Short answerExam level

A car of mass 1000 kg moves at 20 m/s. Find its kinetic energy in joules and kilojoules.

Try it first, then show the solutionSolution

$KE = \frac{1}{2} \times 1000 \times 2 0^{2}$ .
$= 500 \times 400 = 200000$ J.
$= 200$ kJ.

Answer: 200 000 J = 200 kJ

Example 4: PE on a roof · 2 marks · Short answerHarder exam level

A girl of mass 50 kg climbs to a roof 10 m high. Find her gain in potential energy. ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

$PE = m g h = 50 \times 10 \times 10$ .
$= 5000$ J.

Answer: 5000 J

Example 5: PE becomes KE · 3 marks · Short answerHarder exam level

A 0.5 kg ball is dropped from a height of 20 m. Ignoring air resistance, find (i) its PE at the top (ii) its KE just before hitting the ground (iii) its speed then. ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

(i) $PE = 0.5 \times 10 \times 20 = 100$ J.
(ii) All the PE becomes KE: $KE = 100$ J.
(iii) $\frac{1}{2} \times 0.5 \times v^{2} = 100 \Rightarrow v^{2} = 400 \Rightarrow v = 20$ m/s.

Answer: (i) 100 J (ii) 100 J (iii) 20 m/s

Example 6: Power of a crane · 4 marks · Long answerHarder exam level

A crane lifts a load of 200 kg through a height of 15 m in 20 s. Find (i) the work done (ii) the power of the crane. ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

Weight $= 200 \times 10 = 2000$ N.
(i) $W = 2000 \times 15 = 30000$ J.
(ii) $P = \frac{30 000}{20}$ .
$= 1500$ W $= 1.5$ kW.

Answer: (i) 30 000 J (ii) 1500 W

Example 7: Comparing KE and PE · 4 marks · Long answerHOTS / competency

Body A (1 kg) moves at 10 m/s. Body B (1 kg) is held at rest 5 m above the ground. (i) Compare their energies. (ii) What happens to A's energy if its speed is doubled? ( $g = 10 m/s^{2}$ )

Try it first, then show the solutionSolution

A: $KE = \frac{1}{2} \times 1 \times 100 = 50$ J.
B: $PE = 1 \times 10 \times 5 = 50$ J. (i) Equal energies, in different forms.
(ii) At 20 m/s: $KE = \frac{1}{2} \times 1 \times 400 = 200$ J.
Doubling the speed makes the KE four times as large.

Answer: (i) 50 J each (ii) 200 J, four times

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

A small hydroelectric plant

Suppose in a small hydroelectric plant, 1000 kg of water falls through a height of 20 m every second onto a turbine, and half of the energy is changed into electrical energy. Take $g = 10 m/s^{2}$ .

(i) · 2 marks
Find the potential energy given up by 1000 kg of water falling 20 m.
Answer
Answer: 200 000 J
$PE = 1000 \times 10 \times 20 = 200000$ J.
(ii) · 3 marks
How much electrical energy is produced each second, and what is the electrical power output?
Answer
Answer: 100 000 J per second; 100 kW
Half of 200 000 J $= 100000$ J each second. Power $= \frac{100 000}{1} = 100000$ W $= 100$ kW.
(iii) · 3 marks
Write the chain of energy changes from the water in the dam to the electricity.
Answer
Answer: Potential → kinetic → electrical
Stored water has potential energy; as it falls it gains kinetic energy; the moving water turns the turbine and generator, producing electrical energy (some energy becomes heat and sound).

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

A force of 40 N moves a body 3 m in its direction. Find the work done. (2 marks)

Step	Marks
$W = F \times s = 40 \times 3$	1
$= 120$ J	1

A cricket ball of mass 0.2 kg is thrown at 15 m/s. Find its kinetic energy. (3 marks)

Step	Marks
$KE = \frac{1}{2} m v^{2}$	1
$= \frac{1}{2} \times 0.2 \times 225$	1
$= 22.5$ J	1

A boy of mass 60 kg runs up a staircase 4 m high in 8 s. Find the work done against gravity and his power. ( $g = 10 m/s^{2}$ ) (4 marks)

Step	Marks
Weight $= 60 \times 10 = 600$ N	1
$W = 600 \times 4 = 2400$ J	1
$P = \frac{W}{t} = \frac{2400}{8}$	1
$= 300$ W	1

Where students lose marks in Energy

The slips examiners see most often, and what to write instead.

Mistake: Forgetting to square the speed
KE uses $v^{2}$ .
Wrong working
$KE = \frac{1}{2} \times 2 \times 3 = 3$ J
Right working
$\frac{1}{2} \times 2 \times 9 = 9$ J
Do this: Square the speed first, then multiply.
Usually costs: The answer.
Mistake: Using mass instead of weight for lifting work
The force needed is the weight, $m g$ .
Wrong working
$W = 5 \times 2 = 10$ J for a 5 kg bag lifted 2 m
Right working
$W = 50 \times 2 = 100$ J
Do this: Multiply mass by $g = 10$ to get the force.
Usually costs: The answer.
Mistake: Mass in grams
Formulas need kg.
Wrong working
$m = 200$ in the KE formula for a 200 g ball
Right working
$m = 0.2$ kg
Do this: Convert g to kg (÷ 1000).
Usually costs: The answer.
Mistake: Confusing energy and power
Power includes time; energy does not.
Do this: Joule for energy and work; watt for power.
Usually costs: 1 mark.
Mistake: Saying work is done when pushing a fixed wall
There is no displacement, so no work is done on the wall.
Do this: Check for movement in the direction of the force.
Usually costs: 1 mark.

Practice questions on Energy

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQExam level
The energy a body has because of its motion is called:
(a) potential energy
(b) kinetic energy
(c) chemical energy
(d) heat energy
Hint
Motion.
Answer
(b)
Q2 · 1 mark · Very short answerExam level
How many joules are there in 1 kJ?
Hint
Kilo = 1000.
Answer
1000 J.
Q3 · 2 marks · Short answerExam level
Find the work done in lifting a 4 kg box onto a shelf 1.5 m high. ( $g = 10 m/s^{2}$ )
Hint
W = mgh.
Answer
$4 \times 10 \times 1.5 = 60$ J.
Q4 · 3 marks · Short answerHarder exam level
A 2 kg body moves at 6 m/s. Find its KE. What is the KE if (i) the mass is doubled (ii) the speed is doubled?
Hint
½mv².
Answer
36 J; (i) 72 J (ii) 144 J.
Q5 · 1 mark · Assertion-reasonHarder exam level
Assertion (A): A man pushing a wall that does not move does no work on it.
Reason (R): Work is done only when the point of application of the force moves.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
Hint
Displacement.
Answer
(c)
Q6 · 2 marks · Short answerHarder exam level
State the energy changes in (i) a battery-powered torch (ii) an electric fan.
Hint
Start from the source.
Answer
(i) Chemical → electrical → light (and heat). (ii) Electrical → kinetic (with some sound and heat).
Q7 · 2 marks · Short answerHarder exam level
A 4 kg body has 50 J of kinetic energy. Find its speed.
Hint
$v^{2} = \frac{2 \times KE}{m}$ .
Answer
$v^{2} = \frac{100}{4} = 25$ , $v = 5$ m/s.
Q8 · 2 marks · Short answerHarder exam level
A 3 kg stone has 450 J of potential energy. At what height is it? ( $g = 10 m/s^{2}$ )
Hint
h = PE ÷ mg.
Answer
$h = \frac{450}{30} = 15$ m.
Q9 · 2 marks · Short answerHarder exam level
A 750 W motor runs for 1 minute. How much energy does it use?
Hint
Energy = P × t, t in seconds.
Answer
$750 \times 60 = 45000$ J.
Q10 · 2 marks · Short answerHarder exam level
Give two differences between energy and power.
Hint
Definition and unit.
Answer
Energy is the capacity to do work (joule); power is the rate of doing work (watt). Energy does not depend on time; power does.
Q11 · 3 marks · Short answerHOTS / competency
Boy A (400 N) and boy B (500 N) each climb stairs 5 m high. A takes 10 s, B takes 20 s. Who does more work, and who is more powerful?
Hint
Work = weight × height; power = work ÷ time.
Answer
A: 2000 J, 200 W. B: 2500 J, 125 W. B does more work, but A is more powerful.
Q12 · 3 marks · Short answerHOTS / competency
A 2 kg hammer moving at 4 m/s hits a nail and drives it 2 cm into wood. Find the hammer's KE and the average force on the nail, assuming all the KE does work on the nail.
Hint
KE = F × s.
Answer
$KE = \frac{1}{2} \times 2 \times 16 = 16$ J. $F = \frac{16}{0.02} = 800$ N.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Define work.Show answer
Work is done when a force moves a body in the direction of the force; W = F × s.
2.SI unit of energy?Show answer
Joule.
3.Formula for kinetic energy?Show answer
½mv².
4.Formula for gravitational PE?Show answer
mgh.
5.Define power.Show answer
The rate of doing work: P = W/t.
6.SI unit of power?Show answer
Watt (W).
7.Energy change in an electric fan?Show answer
Electrical → kinetic (with some heat and sound).
8.What happens to KE if speed doubles?Show answer
It becomes four times as large.

Best order to study it

Learn the meaning of work and when no work is done.
Practise W = F × s and work against gravity (mgh).
Learn KE = ½mv² and PE = mgh with g = 10.
Learn energy transformations with everyday examples.
Finish with power and the difference between energy and power.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Did you use SI units (kg, m, s)?
Did you square the speed in KE?
Did you use g = 10 m/s²?
Did you check whether there was any displacement?
Can you give three examples of energy changes?
Did you write J for energy and W for power?

Energy: frequently asked questions

Is a person holding a heavy bag still doing work?

Not in the physics sense: the bag does not move, so no work is done on it, even though the person gets tired.

Can a body have both kinetic and potential energy?

Yes. A bird flying above the ground has KE because it moves and PE because it is high up.

Where does the energy go when a ball stops bouncing?

It changes into heat and sound at each bounce and through air resistance; it is not destroyed.

Why is g taken as 10 m/s²?

The actual value is about 9.8 m/s², but the Class VIII syllabus uses 10 m/s² to keep calculations simple.

Is a kilowatt-hour a unit of power?

No. The kilowatt-hour (kWh) is a unit of energy used for electricity bills; it is studied in Electricity.

Based on: CISCE Curriculum for Upper Primary Classes (VI-VIII), Science: Physics (Class VIII); CISCE RDCD: Curriculum Pre-School to Class VIII