PhysicsICSE Class 8 · Energy

Work and Power: W = F × s, P = W/t, Joule and Watt (ICSE Class 8 Physics)

Updated 17 October 2026 · Fact-checked

Work is done when a force moves a body in the direction of the force: W = F × s, in joules (J). Power is the rate of doing work: P = W/t, in watts (1 W = 1 J/s). Energy is the capacity to do work, so it is measured in joules too, but it does not depend on time.

Understand Work, Energy and Power

Work. In physics, work is done only when a force moves a body in the direction of the force:

$W = F \times s$

where $F$ is the force in newton and $s$ the displacement in metre in the direction of the force. The unit is the joule (J): 1 J is the work done when a force of 1 N moves a body through 1 m in its direction.

When no work is done.

the body does not move (pushing a wall that does not shift, holding a heavy bag still);
the motion is at right angles to the force (carrying a bag along a level road does no work against gravity, since the lifting force is vertical and the motion is horizontal).

Work against gravity. To raise a body of mass $m$ steadily through height $h$ , the force needed equals its weight $m g$ (take $g = 10$ m/s², so 1 kg weighs 10 N):

$W = m g \times h$

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

Energy. Energy is the capacity to do work. It is measured in joules, like work. 1 kJ = 1000 J.

Power. Power is the rate of doing work:

$P = \frac{W}{t}$

The unit is the watt (W): 1 W = 1 J/s; 1 kW = 1000 W. A bigger power does the same work in a shorter time. Also, $W = P \times t$ .

Energy	Power
capacity to do work	rate of doing work
unit: joule	unit: watt (J/s)
independent of time taken	depends on time taken

Two people who climb the same stairs do the same work (if their masses are equal), but the one who climbs faster has the greater power.

Work, Energy and Power: formulas and key facts

Work: $W = F \times s$; Joule (J); s along the force.
Work against gravity: $W = m g \times h$; g = 10 m/s²; mass in kg.
Power: $P = \frac{W}{t}$; Watt (W) = J/s; so W = P t.
Units: $1 kJ = 1000 J 1 kW = 1000 W$; Time in seconds for watt.

How to solve Work, Energy and Power questions

Find the work first, then the power, keeping all units SI.

1Check the force and displacement are along the same line; if not, the work in this chapter is zero.
2For lifting, use force = mg (weight) and displacement = height.
3Write $W = F \times s$ and substitute; give the unit joule.
4For power, divide by time in seconds: $P = \frac{W}{t}$ .
5To find time, use $t = \frac{W}{P}$ .

Work, Energy and Power in real life

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.

Carrying water up the stairs

Suppose a girl carries a 12 kg bucket of water up 15 steps of 20 cm each in 12 seconds.

Find the work she does against gravity and her power.

Try it first, then show the solutionSolution

Height $= 15 \times 0.20 = 3$ m.
$W = m g h = 12 \times 10 \times 3 = 360$ J.
$P = \frac{360}{12} = 30$ W.

Answer: 360 J; 30 W.

Memory hook: Weight times height gives work; divide by time for power.

The same job, a faster helper

Imagine two pumps each lift 500 kg of water through 6 m, one taking 100 s and the other 50 s.

Compare the work done and the power of the two pumps.

Try it first, then show the solutionSolution

Work for each $= 500 \times 10 \times 6 = 30000$ J (same).
Power: $\frac{30 000}{100} = 300$ W and $\frac{30 000}{50} = 600$ W.

Answer: Same work; the second pump has twice the power.

Memory hook: Same work, half the time, double the power.

Tricks to remember Work, Energy and Power

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

F-S-W
"Force Spent Where it moves": work only along the displacement.
Use it when: No-work situations.
Joule per second
Watt is just joule/second; every "per second" of work is power.
Use it when: Unit of power.
Lifting = weight × height
m × 10 × h for any lift on Earth in this chapter.
Use it when: Gravity work.

Work, Energy and Power: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Work in physics · 1 mark · MCQBoard level

In which case is work done in the scientific sense?
(a) A boy pushes a wall that does not move
(b) A porter stands holding a suitcase
(c) A girl lifts a bag from the floor to a shelf
(d) A satellite-dish stays fixed on a roof

Try it first, then show the solutionSolution

Work needs a force and displacement along the force. Only in (c) is the bag displaced upwards by the lifting force.

Answer: (c) A girl lifts a bag from the floor to a shelf

Example 2: Lifting a box · 2 marks · Short answerBoard level

A box of 4 kg is raised through 2.5 m. Find the work done against gravity. ( $g = 10$ m/s²)

Try it first, then show the solutionSolution

Force $= m g = 4 \times 10 = 40$ N. (1)
$W = 40 \times 2.5 = 100$ J. (1)

Answer: 100 J

Example 3: Pump on the roof · 3 marks · Short answerHarder board level

A pump lifts 600 kg of water from a well 12 m deep in 2 minutes. Find (a) the work done, (b) the power of the pump. ( $g = 10$ m/s²)

Try it first, then show the solutionSolution

(a) $W = m g h = 600 \times 10 \times 12 = 72000$ J. (1½)
(b) $t = 120$ s; $P = \frac{72 000}{120} = 600$ W. (1½)

Answer: (a) 72 000 J (b) 600 W

Example 4: Who is more powerful? · 4 marks · Long answerHarder board level

Ashok (50 kg) climbs a rope 12 m in 30 s; Bilal (60 kg) climbs the same height in 40 s. Find the work done and power of each. Who has the greater power? ( $g = 10$ m/s²)

Try it first, then show the solutionSolution

Ashok: $W = 50 \times 10 \times 12 = 6000$ J; $P = \frac{6000}{30} = 200$ W. (2)
Bilal: $W = 60 \times 10 \times 12 = 7200$ J; $P = \frac{7200}{40} = 180$ W. (1)
Ashok has the greater power even though Bilal did more work. (1)

Answer: Ashok: 6000 J, 200 W. Bilal: 7200 J, 180 W. Ashok is more powerful.

Example 5: Is work done? · 4 marks · Case-basedHOTS / competency

Rani pushes against a wall for 5 minutes with a force of 300 N; the wall does not move. She then pushes a trolley with 300 N through 4 m in 8 s. (a) Find the work done on the wall. (b) Find the work done on the trolley. (c) Find her power while pushing the trolley. (d) State why (a) is not an exception to the formula.

Try it first, then show the solutionSolution

(a) Displacement $= 0$ , so $W = 300 \times 0 = 0$ . (1)
(b) $W = 300 \times 4 = 1200$ J. (1)
(c) $P = \frac{1200}{8} = 150$ W. (1)
(d) The formula $W = F \times s$ gives zero when $s = 0$ ; effort and tiredness are not work in the physics sense. (1)

Answer: (a) 0 (b) 1200 J (c) 150 W (d) no displacement

Where students lose marks in Work, Energy and Power

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

Mistake: Calling effort "work"
Work needs displacement in the direction of the force.
Wrong working
"Holding a heavy box for 5 minutes does a lot of work."
Right working
"No displacement, so no work is done on the box."
Do this: Check for displacement before using the formula.
Mistake: Mixing units of power
Watt needs seconds; kW needs hours only for kWh.
Wrong working
$P = \frac{72 000}{2} = 36000$ W (time in minutes).
Right working
$P = \frac{72 000}{120} = 600$ W.
Do this: Convert minutes to seconds.
Mistake: Using mass as the force
Force in work is in newton; weight is mg.
Wrong working
$W = 4 \times 2.5 = 10$ J.
Right working
$W = 4 \times 10 \times 2.5 = 100$ J.
Do this: Multiply mass by 10 for the weight.

Exam tips

Write the formula, substitute with units, then give the answer with its unit.
Use g = 10 m/s² unless the question says otherwise.
In "who is more powerful?", compute work and time separately, then divide.
Never confuse energy (J) with power (W): different units.

Practice questions on Work, Energy and Power

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

Q1 · 2 marks · Short answerBoard level
A girl pulls a sledge along level ground with a rope force of 25 N along the direction of motion. It moves 8 m in 10 s. Find the work done and her power.
Hint
$W = F \times s$ , then divide by time.
Answer
$W = 25 \times 8 = 200$ J; $P = \frac{200}{10} = 20$ W.
Q2 · 3 marks · Short answerHarder board level
A motor of power 1.5 kW lifts a 300 kg load through 10 m. How long does it take, if all its power is used for lifting? ( $g = 10$ m/s²)
Hint
Find work, then $t = \frac{W}{P}$ .
Answer
$W = 300 \times 10 \times 10 = 30000$ J; $t = \frac{30 000}{1500} = 20$ s.
Q3 · 3 marks · Short answerHarder board level
A 60 W bulb glows for 2 hours. Find the energy it transforms (a) in joules, (b) in kWh.
Hint
Energy = power × time.
Answer
(a) $60 \times 7200 = 432000$ J. (b) $\frac{60 \times 2}{1000} = 0.12$ kWh.
Q4 · 3 marks · Short answerHarder board level
A man of mass 60 kg climbs 20 steps, each 20 cm high, in 10 s. Find the work done and his power. ( $g = 10$ m/s²)
Hint
Total height = steps × step height.
Answer
Height $= 4$ m; $W = 60 \times 10 \times 4 = 2400$ J; $P = \frac{2400}{10} = 240$ W.
Q5 · 4 marks · Long answerHOTS / competency
A tank needs 2400 kg of water raised 20 m. Pump A has a power of 800 W and pump B of 1.2 kW (assume all their power lifts the water). Find the work needed, the time each takes, and the time difference. Is the electrical energy used different? ( $g = 10$ m/s²)
Hint
Work is the same for both pumps.
Answer
$W = 2400 \times 10 \times 20 = 480000$ J. $t_{A} = \frac{480 000}{800} = 600$ s; $t_{B} = \frac{480 000}{1200} = 400$ s; B is quicker by 200 s. Energy is the same, 480 000 J (about 0.133 kWh) for both: only the rate differs.

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.Formula for work?Show answer
W = F × s.
2.Unit of power?Show answer
Watt (J/s).
3.Work done against gravity?Show answer
mgh.
4.Is work done when a force does not move the body?Show answer
No.
5.Power in terms of work and time?Show answer
P = W/t.

Work, Energy and Power: frequently asked questions

What is work in physics?

Work is done when a force moves a body in the direction of the force. Its value is force × displacement in the direction of the force, measured in joules.

What is the difference between energy and power?

Energy is the capacity to do work, in joules, and does not depend on time. Power is the rate of doing work, in watts (J/s), and depends on the time taken.

What is 1 joule?

One joule is the work done when a force of 1 newton moves a body through 1 metre in the direction of the force.