# A car accelerates from rest along a straight, level road. The engine provides a constant thrust force. Explain why the car's acceleration decreases over time, even though the thrust force remains constant, and describe what happens to the car's motion when the resultant force becomes zero.

> Edexcel GCSE Physics (1PH0) — 9.1 Forces and their effects · Explain · 4 marks

> A car pulls away from a set of traffic lights and accelerates along a straight, level road. The engine provides a constant thrust force throughout.

## Mark scheme (4 marks)

1. As the car speeds up, drag (air resistance) increases
2. The resultant force decreases because thrust remains constant but drag increases, so the forces become more balanced
3. Because resultant force = mass × acceleration (F = ma), a smaller resultant force means smaller acceleration
4. When the resultant force is zero (thrust equals drag plus friction / forces are balanced), the car reaches terminal velocity and travels at a constant speed

## Key terms

- [resultant force](https://www.gradenine.co.uk/glossary/resultant-force)

## Related

- [Revision notes for Edexcel GCSE Physics (1PH0)](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-accelerates-from-rest-along-994e688b) · Published by Druglandscape Ltd.