# Explain why a passenger in a car experiences a sideways force from the car seat when the car travels around a horizontal circular bend at constant speed, even though the passenger's speed is constant.

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

## Mark scheme (4 marks)

1. Velocity is a vector quantity, so changing direction means the velocity is changing even if the speed is constant.
2. A changing velocity means the passenger is accelerating (centripetal acceleration directed towards the centre of the circular path).
3. By Newton's second law, a net force is required to produce this acceleration, directed towards the centre of the circular path.
4. The seat (contact force / normal reaction from the seat) provides this centripetal force, pushing the passenger inwards / towards the centre.

## Related

- [Revision notes for IB DP Physics Higher 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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