# A student pushes a heavy box across a rough horizontal floor at a constant velocity. The student then increases the pushing force. Explain what happens to the motion of the box after the pushing force is increased, and explain why.

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

## Mark scheme (5 marks)

1. At constant velocity, the pushing force equals (is balanced by) the friction force
2. When the pushing force increases, the resultant (net) force is no longer zero / there is a resultant force in the direction of pushing
3. The box accelerates (speeds up)
4. As the box speeds up, the friction force increases (until it equals the new pushing force)
5. Eventually the forces balance again and the box reaches a new (higher) constant velocity / terminal 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-pushes-a-heavy-box-312b9fc3) · Published by Druglandscape Ltd.