# A firefighter stands on a stationary boat floating on a calm lake. The firefighter jumps horizontally off the front of the boat onto a jetty. Explain why the boat moves backwards as the firefighter jumps forwards. Your answer must refer to momentum.

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

> The total mass of the firefighter and boat together is 120 kg. Before the jump, the firefighter and the boat are both stationary.

## Mark scheme (5 marks)

1. Before the jump, the total momentum of the system (firefighter + boat) is zero
2. Momentum is conserved (the total momentum of the system remains the same / is constant)
3. The firefighter gains momentum in the forward direction (as they jump forwards)
4. The boat must gain an equal and opposite momentum (in the backward direction) so the total remains zero
5. Because the boat has a larger mass than the firefighter, it moves backwards more slowly (lower speed than the firefighter's forward speed)

## Key terms

- [momentum](https://www.gradenine.co.uk/glossary/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-firefighter-stands-on-a-stationary-06958050) · Published by Druglandscape Ltd.