# A railway wagon of mass 4000 kg is moving at 3 m/s along a level track when it collides with a stationary wagon of mass 2000 kg. After the collision the two wagons join together and move as one. Explain what happens to the total momentum of the two wagons during this collision, and describe how the velocity of the joined wagons after the collision compares with the velocity of the first wagon before the collision.

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

> Momentum is defined as the product of an object's mass and its velocity. The principle of conservation of momentum states that, provided no external forces act, the total momentum of a system before a collision equals the total momentum after the collision.

## Mark scheme (5 marks)

1. The total momentum of the system is conserved (stays the same / does not change) during the collision
2. This is because no external (resultant) force acts on the system / the track is level and smooth
3. The total mass after the collision is greater than the mass of the first wagon alone (combined mass is 6000 kg)
4. Because momentum is conserved and mass has increased, the velocity of the joined wagons must be less than 3 m/s
5. The velocity of the joined wagons is 2 m/s (correctly applying conservation of momentum: 4000 × 3 = 6000 × v, so v = 2 m/s)

## 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)
- [collision](https://www.gradenine.co.uk/glossary/collision)

## 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-railway-wagon-of-mass-4000-1a6972d2) · Published by Druglandscape Ltd.