# A skateboarder standing still on a smooth, flat surface throws a heavy ball horizontally away from them. After throwing the ball, the skateboarder moves in the opposite direction to the ball. Using the concept of conservation of momentum, explain why the skateboarder moves after releasing the ball, and describe what determines how fast the skateboarder moves.

> Eduqas A-Level Physics — 1.4 Momentum (A-Level only) · Explain · 5 marks

> Before the throw, the skateboarder and ball are both stationary. The skateboarder has a mass of 60 kg. The ball is thrown horizontally.

## Mark scheme (5 marks)

1. The total momentum of the skateboarder and ball before the throw is zero (system is stationary).
2. By conservation of momentum, the total momentum after the throw must also equal zero.
3. The ball gains momentum in one direction, so the skateboarder must gain an equal and opposite momentum to keep the total at zero.
4. The speed of the skateboarder depends on the mass and speed of the thrown ball — a heavier or faster-thrown ball causes the skateboarder to move faster.
5. The speed of the skateboarder is also affected by the skateboarder's own mass — a greater mass of skateboarder means a lower speed for the same ball momentum.

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Physics](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-skateboarder-standing-still-on-a-8ceee848) · Published by Druglandscape Ltd.