# Explain why the total momentum of a system is conserved when two objects collide.

> AQA GCSE Physics (8463) — 4.5.7 Momentum (Phys only) · Explain · 4 marks

> Two ice skaters push off from each other whilst standing still on an ice rink. After they push, they move in opposite directions.

## Mark scheme (4 marks)

1. Before the event, both skaters are stationary, so the total momentum of the system is zero
2. The skaters exert equal and opposite forces on each other (Newton's Third Law)
3. After the push, the skaters move in opposite directions, so their momenta are in opposite directions
4. The momenta cancel / add to zero, so total momentum after the event is still zero, equalling the total momentum before

## Key terms

- [momentum](https://www.gradenine.co.uk/glossary/momentum)

## Related

- [Revision notes for AQA GCSE Physics (8463)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-total-momentum-of-24a04191) · Published by Druglandscape Ltd.