# A skateboarder standing still on a flat, frictionless surface pushes against a wall. After pushing, the skateboarder moves away from the wall. Explain, using Newton's laws of motion and the concept of momentum, why the skateboarder moves away from the wall and what happens to the skateboarder's speed once they have left the wall.

> OCR A-Level Physics B: Advancing Physics (H557) — 3.5 Newton's laws and momentum · Explain · 5 marks

> A skateboarder with a mass of 60 kg stands stationary on a frictionless surface and pushes against a fixed wall. After the push, the skateboarder rolls away from the wall.

## Mark scheme (5 marks)

1. The skateboarder and wall exert equal and opposite forces on each other (Newton's third law / action-reaction pair)
2. The resultant force on the skateboarder causes them to accelerate / gain momentum in the direction away from the wall (Newton's second law)
3. Before the push, the total momentum of the system is zero as both the skateboarder and wall are stationary
4. The skateboarder gains momentum in one direction; the wall (and Earth) gains equal momentum in the opposite direction, so total momentum is conserved (conservation of momentum)
5. Once the skateboarder leaves the wall there is no resultant force (frictionless surface), so by Newton's first law the skateboarder continues at constant speed

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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-62cd1d3b) · Published by Druglandscape Ltd.