# Energy, work and power

> Work done by a force, kinetic and gravitational potential energy, conservation of energy, power and efficiency, including work from a force–displacement graph.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.energy-power
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 1.4
- Difficulty: 2/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 1.4(a) | calculate work done as force times displacement resolved along the direction of motion | AO2 |
| Unit 1.4(b) | use kinetic and gravitational potential energy and conservation of mechanical energy | AO2 |
| Unit 1.4(c) | calculate power and efficiency for mechanical systems | AO1, AO2 |
| Unit 1.4(d) | determine work from the area under a force-displacement graph | AO2 |
| Unit 1.4(e) | solve problems involving gravitational potential energy m g h and elastic potential energy | AO2 |
| Unit 1.4(f) | analyse energy transfers in systems with non-conservative forces and efficiency | AO2, AO3 |

## Summary

Work done by a force, kinetic and gravitational potential energy, conservation of energy, power and efficiency, including work from a force–displacement graph.

## What earns marks

- W = Fs cosθ — only the component of force along the displacement does work.
- Efficiency = useful output ÷ total input, always ≤ 1.
- P = Fv for a constant force at constant velocity.
- The area under a force–displacement graph is the work done.

## Where marks are lost

- Omitting cosθ when the force is at an angle.
- Quoting an efficiency above 100% after a unit slip.
- Saying energy is 'lost' rather than dissipated, usually as heat.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Energy, work and power lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.energy-power)