# A shopping trolley is pushed from rest along a flat supermarket car park. The person pushing the trolley applies a constant horizontal force. After several seconds, the trolley reaches a constant (terminal) velocity. Explain why the trolley reaches a constant velocity even though the pushing force remains the same.

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

## Mark scheme (5 marks)

1. As the trolley speeds up, the resistive forces (friction/air resistance) acting on it increase
2. At first, the driving/pushing force is greater than the resistive forces, so there is a resultant force
3. The resultant force causes the trolley to accelerate (Newton's second law)
4. Eventually the resistive forces increase until they equal the pushing force
5. When the two forces are equal, the resultant force is zero, so the trolley moves at constant velocity

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-shopping-trolley-is-pushed-from-fc39793b) · Published by Druglandscape Ltd.