A motor does 6000\,J of work in 20\,s. Calculate its power. [2 marks]

WJEC WJEC 2019-style practice: A crane lifts a 500\,kg load to a height of 12\,m. Calculate the gain in gravitational potential energy (g = 10\,N/kg).

A topic 2.3 Calculate question. Use E_p = mgh (1 mark). Substitute m = 500\,kg, g = 10\,N/kg and h = 12\,m: E_p = 500 × 10 × 12 (1 mark) = 60\,000\,J (1 mark for the answer with units). Markers reward the equation, the substitution and the unit joules. A common error is to leave out g.

WalesPhysicsSyllabus dot point

How are work, energy and power calculated, and how does a spring store energy?

Work done, power, kinetic and gravitational potential energy, energy conservation, and Hooke's law for springs.

A focused answer to WJEC GCSE Physics topic 2.3 on work and energy, covering work done, power, kinetic and gravitational potential energy, the conservation of energy, and Hooke's law for the extension of a spring.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-14

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is asking
Work and power
Kinetic and potential energy
Conservation of energy and Hooke's law
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What this topic is asking

WJEC wants you to calculate work done, power, kinetic energy and gravitational potential energy, apply conservation of energy, and use Hooke's law for springs. This is topic 2.3 Work and energy in Unit 2 of WJEC GCSE Physics (3420).

Work and power

Work and energy are measured in the same unit, the joule, because doing work transfers energy. A more powerful device transfers the same energy in less time, or more energy in the same time. For example, two cranes that lift the same load to the same height do the same work, but the more powerful crane does it faster. When a force does work against friction, the energy is transferred to the surroundings as heat, which is why brakes and rubbed surfaces warm up.

It is worth being careful with the word "work" in physics: work is done only when a force moves something in the direction of the force. Holding a heavy bag still does no work in the physics sense, even though it feels tiring, because the bag does not move. If you carry the bag horizontally, the upward force you apply does no work either, because the movement is at right angles to the force.

Kinetic and potential energy

Conservation of energy and Hooke's law

Investigating how the extension of a spring depends on the force (force and extension for a spring) is a specified WJEC practical. Hanging known weights and measuring the extension gives a straight line through the origin while the spring obeys Hooke's law; the gradient is the spring constant.

Falling object: potential to kinetic energy

A $2.0\,\text{kg}$ ball is dropped from a height of $5.0\,\text{m}$ . Assuming no energy is lost, find its speed just before it lands ( $g = 10\,\text{N/kg}$ ).

step 1: Find the potential energy lost

$E_p = mgh = 2.0 \times 10 \times 5.0 = 100\,\text{J}$ .

step 2: Equate it to kinetic energy gained

All the potential energy becomes kinetic energy, so $E_k = 100\,\text{J} = \tfrac{1}{2}mv^2$ .

step 3: Rearrange for the speed

$100 = \tfrac{1}{2} \times 2.0 \times v^2$ , so $100 = v^2$ , giving $v = \sqrt{100}$ .

step 4: State the answer

$v = 10\,\text{m/s}$ . Energy conservation lets you swap potential energy for kinetic energy without using the equations of motion.

Try this

Q1. A motor does $6000\,\text{J}$ of work in $20\,\text{s}$ . Calculate its power. [2 marks]

Cue. $P = \dfrac{W}{t} = \dfrac{6000}{20} = 300\,\text{W}$ .

Q2. State what is meant by the spring constant. [1 mark]

Cue. The force per unit extension of a spring (the gradient of a force-extension graph).

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20193 marksA crane lifts a

500\,\text{kg}

load to a height of

12\,\text{m}

. Calculate the gain in gravitational potential energy (

g = 10\,\text{N/kg}

Show worked answer →

A topic 2.3 Calculate question. Use $E_p = mgh$ (1 mark). Substitute $m = 500\,\text{kg}$ , $g = 10\,\text{N/kg}$ and $h = 12\,\text{m}$ : $E_p = 500 \times 10 \times 12$ (1 mark) $= 60\,000\,\text{J}$ (1 mark for the answer with units). Markers reward the equation, the substitution and the unit joules. A common error is to leave out $g$ .

WJEC 20214 marksA spring obeys Hooke's law. Describe how its extension changes as the load increases, and state when this stops being true.

Show worked answer →

A topic 2.3 Describe question. While the spring obeys Hooke's law, the extension is directly proportional to the load: doubling the force doubles the extension (1 mark), so a graph of force against extension is a straight line through the origin (1 mark). This holds up to the limit of proportionality (elastic limit) (1 mark); beyond it the line curves and the spring may not return to its original length (1 mark). Markers reward proportionality, the straight-line graph and the limit. A common error is to say the spring always returns to shape.

Related dot points

Sources & how we know this

WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)