# A rocket is launched from rest into space. Once in deep space, far from any gravitational fields, the rocket engines are switched off. The rocket continues to move at a constant velocity. A small thruster then fires briefly, pushing the rocket sideways. Explain, using Newton's laws of motion and the concept of momentum, why the rocket continues at constant velocity when the engines are off, and what happens to the rocket's momentum when the thruster fires.

> OCR A-Level Physics B: Advancing Physics (H557) — 3.5 Newton's laws and momentum · Explain · 5 marks

> A rocket travelling in deep space has its main engines switched off. It continues moving at a constant velocity. A small thruster then fires briefly, exerting a force on the rocket for a short time.

## Mark scheme (5 marks)

1. When the engines are off, there are no unbalanced (resultant) forces acting on the rocket in deep space
2. By Newton's first law, an object continues at constant velocity (or remains at rest) unless acted on by a resultant force
3. When the thruster fires, it exerts a resultant force on the rocket
4. By Newton's second law (or the relationship between force, mass and acceleration), this force causes the rocket to accelerate / change velocity
5. The rocket's momentum changes because momentum = mass × velocity, and the force from the thruster causes a change in velocity (and therefore a change in momentum)

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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-rocket-is-launched-from-rest-6d40dc41) · Published by Druglandscape Ltd.