# A rocket in deep space fires its engines, ejecting hot gases backwards. Explain, using Newton's laws of motion and the concept of momentum, why the rocket accelerates forwards when the engines fire, and why the rocket continues to move at a constant velocity after the engines are switched off.

> OCR A-Level Physics A (H556) — 3.5 Newton's laws and momentum · Explain · 5 marks

> A rocket travelling through deep space is far from any significant gravitational fields. When its engines fire, hot gases are expelled backwards at high speed. The engines are then switched off and the rocket continues to travel through space.

## Mark scheme (5 marks)

1. The total momentum of the rocket-gas system before the engines fire is zero (rocket is stationary) / momentum is conserved in the system
2. The gases are given momentum in the backwards direction, so by conservation of momentum the rocket gains equal and opposite momentum forwards
3. By Newton's third law, the gases exert an equal and opposite force on the rocket to the force the rocket exerts on the gases
4. The resultant forward force on the rocket causes it to accelerate forwards, in accordance with Newton's second law (resultant force = mass × acceleration)
5. Once the engines are off, there is no resultant force on the rocket (deep space — no air resistance or gravity), so by Newton's first law it continues at constant velocity

## 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 A (H556)](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-in-deep-space-fires-c06c899f) · Published by Druglandscape Ltd.