# Explain why a passenger in a car experiences a forward lurch when the car brakes suddenly to a stop.

> IB DP Physics Standard Level (2023 syllabus) — A.2 Forces and momentum · Explain · 4 marks

> A car travelling at constant velocity brakes sharply and comes to rest in a short time interval.

## Mark scheme (4 marks)

1. Before braking, the passenger is moving at the same velocity as the car.
2. A net (resultant) force is required to change the momentum of an object (Newton's second law / impulse-momentum theorem).
3. The car decelerates but no sufficient horizontal force acts on the passenger to decelerate them at the same rate as the car.
4. The passenger tends to continue moving forward (at the original velocity) relative to the car, so they appear to lurch forward relative to the decelerating car.

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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