# A lorry travelling at high speed along a motorway collides with a stationary car. After the collision, both vehicles move together in the same direction. Explain, using Newton's laws of motion and the concept of momentum, why the lorry slows down during the collision and why the combined speed of the vehicles after the collision is much lower than the lorry's original speed.

> OCR A-Level Physics A (H556) — 3.5 Newton's laws and momentum · Explain · 5 marks

> A lorry of mass 10 000 kg is travelling at 30 m/s along a motorway. It collides with a stationary car of mass 1 500 kg. After the collision the two vehicles move together.

## Mark scheme (5 marks)

1. During the collision, the lorry exerts a force on the car (Newton's third law: the car exerts an equal and opposite force back on the lorry)
2. The resultant (backwards) force on the lorry causes it to decelerate / slow down (Newton's second law: resultant force causes a change in momentum)
3. Momentum is conserved in the collision (total momentum before = total momentum after)
4. Before the collision only the lorry has momentum; the car is stationary so its momentum is zero
5. After the collision the same total momentum is shared by a greater combined mass, so the combined speed is much lower than the lorry's original speed

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics A (H556)](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-lorry-travelling-at-high-speed-01c414ac) · Published by Druglandscape Ltd.