# Explain why a rocket accelerates even though it is in empty space where there is no medium to push against.

> IB DP Physics Higher Level (2023 syllabus) — A.2 Forces and momentum · Explain · 4 marks

> A rocket engine works by expelling hot exhaust gases at high speed from its nozzle. Students sometimes believe that a rocket needs air or some external medium to 'push against' in order to accelerate.

## Mark scheme (4 marks)

1. The rocket and exhaust gases form a closed/isolated system, so the total momentum of the system is conserved.
2. The exhaust gases are expelled backwards (rearwards), gaining momentum in that direction.
3. By Newton's third law / conservation of momentum, the rocket gains an equal and opposite momentum in the forward direction.
4. Because the rocket gains momentum over time, there is a net force (thrust) on the rocket, causing it to accelerate in the forward direction.

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](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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