# A railway carriage of mass 4000 kg is moving at 3 m/s along a level track. It collides with a stationary carriage of mass 2000 kg. After the collision the two carriages join together and move as one. Explain what happens to the total momentum of the two carriages during the collision, and describe how the velocity of the joined carriages after the collision compares to the velocity of the moving carriage before the collision. Give reasons for your answers.

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

## Mark scheme (5 marks)

1. The total momentum of the two carriages is conserved / stays the same / does not change during the collision
2. This is because no external (resultant) force acts on the system / it is a closed system
3. The total mass after the collision is greater than the mass of the moving carriage alone
4. Because momentum is conserved and the total mass has increased, the velocity after the collision is lower / smaller than 3 m/s
5. The velocity of the joined carriages is 2 m/s (correct value obtained by applying conservation of momentum: 4000 × 3 = 6000 × v)

## Key terms

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

## 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-railway-carriage-of-mass-4000-2755d0e7) · Published by Druglandscape Ltd.