# A car travelling at high speed collides with a stationary barrier and comes to rest. Explain, using Newton's laws of motion and the concept of momentum, why a longer collision time reduces the force experienced by the car's occupants.

> OCR A-Level Physics B: Advancing Physics (H557) — 3.5 Newton's laws and momentum · Explain · 5 marks

> Modern cars are designed with crumple zones at the front and rear. These crumple zones are designed to increase the time taken for the car to stop during a collision.

## Mark scheme (5 marks)

1. The car (and occupants) have momentum before the collision / momentum = mass × velocity
2. During the collision the momentum changes / the car goes from moving to stationary so its momentum changes to zero
3. By Newton's second law, force is related to the rate of change of momentum / force equals change in momentum divided by time
4. The change in momentum is the same regardless of collision time (same initial and final momentum)
5. A longer collision time means the rate of change of momentum is smaller, so the force on the occupants is smaller / crumple zones increase collision time and therefore reduce the force

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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-car-travelling-at-high-speed-b9995212) · Published by Druglandscape Ltd.