# Momentum and collisions

> Linear momentum, impulse, conservation of momentum in one and two dimensions, and elastic versus inelastic collisions.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.momentum
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 3.1
- Difficulty: 3/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 3.1(a) | define linear momentum and impulse as force multiplied by time | AO1 |
| Unit 3.1(b) | apply conservation of linear momentum to collisions and explosions in one dimension | AO2 |
| Unit 3.1(c) | distinguish elastic collisions where kinetic energy is conserved from inelastic where it is not | AO1 |
| Unit 3.1(d) | use Newton second law as rate of change of momentum to determine average force | AO2 |
| Unit 3.1(e) | analyse two-dimensional collisions by resolving momentum into perpendicular components | AO2 |
| Unit 3.1(f) | evaluate experimental evidence for momentum conservation including uncertainties | AO3 |

## Summary

Linear momentum, impulse, conservation of momentum in one and two dimensions, and elastic versus inelastic collisions.

## What earns marks

- Momentum is conserved in any collision in a closed system; kinetic energy is conserved only in elastic ones.
- Impulse = FΔt = Δp = the area under a force–time graph.
- Momentum is a vector — assign and keep a sign convention.
- In two dimensions, conserve momentum in each perpendicular direction separately.

## Where marks are lost

- Assuming kinetic energy is conserved in every collision.
- Dropping the direction sign for a rebounding object, halving the calculated impulse.
- Using speed rather than velocity components in 2D problems.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Momentum and collisions lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.momentum)