# A car is travelling along a straight, level road at a constant velocity of 20 m/s. The driver sees an obstacle ahead and applies the brakes. The car decelerates uniformly and comes to rest. Explain, in terms of forces, why the car travels at constant velocity before the brakes are applied, and then explain why the car decelerates once the brakes are applied.

> AQA A-Level Physics (7408) — 3.4.1 Mechanics · Explain · 5 marks

## Mark scheme (5 marks)

1. At constant velocity, the driving force equals the resistive forces / friction and air resistance
2. Because the resultant force is zero, there is no acceleration (Newton's first law)
3. When brakes are applied, a braking force / friction acts on the car (in the opposite direction to motion)
4. This produces a resultant force in the opposite direction to motion
5. A resultant force causes acceleration (deceleration), so the car slows down and stops

## Key terms

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

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-car-is-travelling-along-a-e343b707) · Published by Druglandscape Ltd.