# A person stands on a stationary rowing boat that is floating on a calm lake. The person jumps horizontally off the front of the boat onto a jetty. Explain what happens to the boat when the person jumps, using the principle of conservation of momentum.

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

> The lake surface is smooth enough that friction between the boat and the water can be ignored. Before the jump, both the person and the boat are completely stationary.

## Mark scheme (5 marks)

1. Before the jump the total momentum of the system (person + boat) is zero
2. The total momentum of the system must remain zero after the jump (conservation of momentum)
3. The person gains momentum in the forward direction (towards the jetty)
4. The boat must gain an equal and opposite momentum (moves away from the jetty / backwards)
5. The boat's momentum is equal in magnitude to the person's momentum so that the total remains zero

## 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 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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Source: [GradeNine](https://www.gradenine.co.uk/q/a-person-stands-on-a-stationary-89ac6187) · Published by Druglandscape Ltd.