# A ice skater of mass 60 kg is moving at 4 m/s across a frictionless ice rink when she collides with a stationary skater of mass 40 kg. After the collision, the two skaters move together. State the principle of conservation of momentum and use it to explain what happens to the velocity of the pair of skaters after the collision compared to the initial velocity of the moving skater.

> Cambridge International IGCSE Physics (0625) — 1.6 Momentum · State and Explain · 4 marks

> A 60 kg ice skater moving at 4 m/s collides with a stationary 40 kg skater on a frictionless surface. After the collision, the two skaters move together as one combined mass of 100 kg.

## Mark scheme (4 marks)

1. States that the total momentum before a collision equals the total momentum after a collision (in the absence of external forces)
2. States that momentum is a product of mass and velocity, so the combined (greater) mass must move at a lower velocity to conserve momentum
3. Correctly identifies that the total momentum before the collision is 240 kg m/s (60 kg × 4 m/s)
4. Correctly concludes that the velocity of the combined pair after the collision is less than 4 m/s (e.g. 2.4 m/s), because the same momentum is now shared by a greater total mass

## Key terms

- [momentum](https://www.gradenine.co.uk/glossary/momentum)
- [conservation of momentum](https://www.gradenine.co.uk/glossary/conservation-of-momentum)
- [velocity](https://www.gradenine.co.uk/glossary/velocity)
- [mass](https://www.gradenine.co.uk/glossary/mass)

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
- [How to answer "State and 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-ice-skater-of-mass-60-0415b26a) · Published by Druglandscape Ltd.