# A car accelerates from rest along a straight, level road. The car's engine provides a constant thrust force. Explain why the car first accelerates rapidly, then accelerates more slowly, and finally reaches a constant speed.

> Edexcel A-Level Physics (9PH0) — 9.1 Kinetic theory and ideal gases · Explain · 5 marks

> A car's engine provides a constant thrust force. At first, the car speeds up quickly. Over time, the acceleration decreases. Eventually, the car travels at a constant speed.

## Mark scheme (5 marks)

1. At the start, friction and drag (air resistance) are small, so the resultant force is large
2. A large resultant force causes a large acceleration (linked to F = ma)
3. As speed increases, air resistance / drag increases
4. Increasing drag reduces the resultant force, so acceleration decreases
5. Terminal velocity is reached when thrust equals drag and friction, giving a zero resultant force and no acceleration, so constant speed

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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