# A football of mass 0.45 kg is moving at 12 m/s when it strikes a goalkeeper's hands and comes to rest. State what is meant by the principle of conservation of momentum and use it to explain what happens to the momentum of the football–goalkeeper system during this collision.

> Cambridge International IGCSE Physics (0625) — 1.6 Momentum · State and Explain · 4 marks

> A goalkeeper of mass 75 kg is initially standing still on a football pitch. A football of mass 0.45 kg travels towards the goalkeeper at 12 m/s. The goalkeeper catches the ball and both come to rest together immediately after the catch.

## Mark scheme (4 marks)

1. The principle of conservation of momentum states that the total momentum of a system remains constant / is conserved, provided no external force acts on it.
2. The football has momentum before the collision (momentum = mass × velocity, in the direction of travel).
3. The goalkeeper is initially stationary, so the goalkeeper's initial momentum is zero; therefore the total initial momentum of the system equals only the football's momentum.
4. After the catch, both the ball and goalkeeper are at rest, so the final momentum of the system is zero; this is consistent with conservation of momentum because an external force (friction / ground reaction) acts to bring the system to rest, meaning the system is not closed/isolated.

## Key terms

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

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
- [How to answer "State and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-football-of-mass-0-45-kg-ec21f503) · Published by Druglandscape Ltd.