# A car is travelling along a straight, level road at a constant speed. The driver then presses the accelerator pedal and the car speeds up. Explain, using Newton's laws, why the car travels at a constant speed before the accelerator is pressed, and why it then accelerates when the driver presses the accelerator pedal.

> WJEC A-Level Physics (Wales) — 1.2 Forces and Newton's laws · Explain · 5 marks

## Mark scheme (5 marks)

1. At constant speed, the driving force equals the resistive forces (e.g. friction/air resistance)
2. When forces are balanced the resultant force is zero
3. Newton's first law: an object continues at constant velocity (or remains stationary) unless acted on by a resultant force
4. When the accelerator is pressed, the driving force increases so the driving force is greater than the resistive forces, producing a resultant force
5. Newton's second law: a resultant force causes acceleration (in the direction of the resultant force)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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-is-travelling-along-a-5104456e) · Published by Druglandscape Ltd.