# A child standing on a stationary ice rink pushes a heavy sledge horizontally away from them. After the push, the child moves in the opposite direction to the sledge. Use the principle of conservation of momentum to explain why the child moves backwards after releasing the sledge.

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

> Before the push, both the child and the sledge are completely at rest on the ice. The ice is smooth, so friction can be ignored.

## Mark scheme (5 marks)

1. The total momentum of the child and sledge before the push is zero (because both are stationary / at rest)
2. Conservation of momentum states that total momentum is the same before and after the push (in a closed / isolated system)
3. The sledge gains momentum in one direction (away from the child)
4. The child must gain an equal and opposite momentum (in the other direction) so that the total remains zero
5. This means the child moves in the opposite direction to the sledge

## 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 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-child-standing-on-a-stationary-a725843e) · Published by Druglandscape Ltd.