# A firefighter standing on a smooth, frictionless platform fires a water hose horizontally. The jet of water travels in one direction and the firefighter and platform begin to move in the opposite direction. Explain why the firefighter and platform move, and describe what happens to the speed of the firefighter and platform as they continue to move while water is being fired.

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

> A firefighter stands on a smooth, frictionless platform on wheels. When the firefighter opens the hose and a continuous jet of water is fired horizontally, the firefighter and platform begin to move in the opposite direction to the water.

## Mark scheme (5 marks)

1. The total momentum of the system before the hose is fired is zero (the system is stationary / at rest).
2. The law of conservation of momentum states that the total momentum of a closed system remains constant / is conserved.
3. As the water gains momentum in one direction, the firefighter and platform must gain equal momentum in the opposite direction so that total momentum remains zero.
4. While water continues to be fired, the firefighter and platform continue to gain momentum / accelerate (their speed increases).
5. The greater the rate at which water is fired (greater mass of water per second or higher speed of water), the greater the rate of change of momentum of the firefighter and platform / the faster they accelerate.

## 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-firefighter-standing-on-a-smooth-876f9fde) · Published by Druglandscape Ltd.