# Explain why two ice skaters, initially at rest, move in opposite directions after pushing off from each other.

> IB DP Physics Standard Level (2023 syllabus) — A.2 Forces and momentum · Explain · 4 marks

> Two ice skaters stand face-to-face on a frictionless ice rink. Skater A has a greater mass than Skater B. They push off from each other and move apart.

## Mark scheme (4 marks)

1. The total momentum of the system before the push is zero, so the total momentum must remain zero after the push (conservation of momentum / no external horizontal force acts on the system).
2. By Newton's third law, the force Skater A exerts on Skater B is equal in magnitude and opposite in direction to the force Skater B exerts on Skater A.
3. Since the forces are equal and opposite, the momenta gained are equal in magnitude and opposite in direction, so the skaters move in opposite directions.
4. Because Skater A has greater mass, Skater A moves with a smaller speed than Skater B (since momentum = mass × velocity and both momenta are equal in magnitude).

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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