# A ball of clay travelling horizontally at 4 m/s collides with an identical stationary ball of clay. The two balls stick together and move off as one combined mass. Explain, using the principle of conservation of momentum, why the combined mass moves off at a lower speed than 4 m/s.

> Pearson Edexcel International GCSE Physics (4PH1) — 1.3 Forces, movement, shape and momentum · Explain · 4 marks

> In a physics investigation, two identical balls of clay, each of mass 0.5 kg, are used. One ball is moving at 4 m/s and the other is stationary. They collide and stick together.

## Mark scheme (4 marks)

1. The total momentum before the collision equals the total momentum after the collision (conservation of momentum).
2. Before the collision, only one ball has momentum; the stationary ball has zero momentum, so total momentum = mass × 4 m/s.
3. After the collision, the combined (larger) mass carries the same total momentum.
4. Because momentum = mass × velocity, if mass increases and momentum stays the same, the velocity must decrease / the combined mass moves more slowly.

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-ball-of-clay-travelling-horizontally-c5dbe654) · Published by Druglandscape Ltd.