# A wooden crate is pushed across a rough concrete warehouse floor at a constant velocity by a worker applying a horizontal force. Explain why the worker must continue to apply a force to keep the crate moving at constant velocity, and describe what would happen to the motion of the crate if the worker suddenly doubles the pushing force.

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

> A wooden crate of weight 400 N is pushed across a rough concrete warehouse floor. The worker applies a constant horizontal force and the crate moves at a steady speed.

## Mark scheme (5 marks)

1. Friction acts on the crate in the opposite direction to motion (because the surfaces are rough / in contact)
2. At constant velocity the forces are balanced / resultant force is zero, so the applied force must equal friction
3. If the worker applied no force, friction would decelerate / slow the crate (because there would be a resultant force in the direction of friction)
4. When the applied force is doubled, the pushing force is greater than friction, so there is a resultant force in the direction of motion
5. The resultant force causes the crate to accelerate (in the direction of the applied force)

## Key terms

- [constant velocity](https://www.gradenine.co.uk/glossary/constant-velocity)

## 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)
- [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-wooden-crate-is-pushed-across-524d2d93) · Published by Druglandscape Ltd.