# A car travelling at high speed brakes suddenly and comes to a stop. Explain, using Newton's laws of motion, why the passengers continue to move forwards after the car stops, and describe how seat belts reduce the risk of injury to passengers.

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

> Modern cars are fitted with seat belts designed to keep passengers safe during sudden deceleration. Safety engineers consider both the forces acting on passengers and the time over which those forces act when designing restraint systems.

## Mark scheme (5 marks)

1. Newton's First Law: passengers tend to remain in their state of motion (continue moving forwards) when the car stops, because no resultant force is acting on them to change their motion
2. The car decelerates but no force has yet acted on the passenger to decelerate them, so they continue at the original speed
3. Newton's Second Law / the seat belt exerts a resultant force on the passenger to decelerate them (force causes change in velocity / acceleration)
4. The seat belt stretches / increases the time over which the passenger decelerates
5. A longer stopping time means a smaller force acts on the passenger (for the same change in momentum), reducing injury

## 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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