# A skateboarder stands still on a skateboard on a smooth, flat surface. The skateboarder throws a heavy bag horizontally. After throwing the bag, the skateboarder and skateboard move in the opposite direction. Explain why the skateboarder moves after throwing the bag, and describe what happens to the speed of the skateboarder compared to the speed of the bag.

> Edexcel A-Level Physics (9PH0) — 2.2 Forces, Newton's laws and momentum · Explain · 5 marks

> A skateboarder of mass 60 kg stands on a skateboard on a smooth, flat surface. The skateboarder throws a heavy bag of mass 10 kg horizontally. Before throwing the bag, both the skateboarder and the bag are stationary.

## Mark scheme (5 marks)

1. Before the throw, the total momentum of the system is zero (system is stationary)
2. The total momentum must remain zero after the throw because momentum is conserved
3. The bag gains momentum in one direction, so the skateboarder must gain equal momentum in the opposite direction
4. The skateboarder has a greater mass than the bag
5. Because momentum = mass × velocity, the skateboarder moves more slowly than the bag

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](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-stands-still-on-a-a1536614) · Published by Druglandscape Ltd.