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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
The total momentum of the child and sledge before the push is zero, because both are at rest. Conservation of momentum states that the total momentum of a closed system remains constant, so the total after the push must also be zero. The sledge acquires a positive momentum in the direction it is pushed. To keep the total momentum zero, the child must acquire an equal magnitude of momentum in the opposite direction. Therefore the child moves backwards.
Examiner tips
- State the initial total momentum is zero. Mention conservation of momentum. Explain the sledge’s momentum. Show the child’s opposite momentum. Conclude the child moves backwards.
Common mistakes
- Failing to note that the initial momentum is zero. Using the wrong sign for the child’s momentum. Not linking conservation of momentum to the child’s motion.
Mark scheme (5 marks)
- The total momentum of the child and sledge before the push is zero (because both are stationary / at rest)
- Conservation of momentum states that total momentum is the same before and after the push (in a closed / isolated system)
- The sledge gains momentum in one direction (away from the child)
- The child must gain an equal and opposite momentum (in the other direction) so that the total remains zero
- This means the child moves in the opposite direction to the sledge
Key terms in this question
momentum · conservation of momentum
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