# A skateboarder is standing still on a smooth, flat surface. The skateboarder throws a heavy ball horizontally away from them. After releasing the ball, the skateboarder moves in the opposite direction. Explain why the skateboarder moves after releasing the ball, and describe how the speed of the skateboarder compares to the speed of the ball immediately after release.

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

> A skateboarder of mass 60 kg stands stationary on a skateboard on a smooth, flat surface. The skateboarder throws a ball of mass 3 kg horizontally.

## Mark scheme (5 marks)

1. Before the throw, the total momentum of the skateboarder and ball system is zero (the system is stationary).
2. Momentum is conserved (the total momentum of the system remains zero after the throw).
3. The ball gains momentum in one direction, so the skateboarder must gain an equal and opposite momentum, causing the skateboarder to move in the opposite direction.
4. The skateboarder moves more slowly than the ball because the skateboarder has a much greater mass than the ball.
5. The momentum of the skateboarder equals the momentum of the ball (momentum = mass × velocity), so for a larger mass the velocity must be smaller.

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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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