# Explain why a rocket moving through empty space can accelerate even though there is no medium for it to push against.

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

> A rocket in deep space fires its engines by expelling hot exhaust gases at high speed from its rear nozzle. The rocket's speed is observed to increase as a result.

## Mark scheme (4 marks)

1. The total momentum of the rocket–exhaust system is conserved (or: the initial total momentum is zero / constant).
2. The exhaust gases are given momentum in one direction (rearward), so by conservation of momentum the rocket gains an equal and opposite momentum in the forward direction.
3. By Newton's third law, the force the rocket exerts on the expelled gases (rearward) is equal in magnitude and opposite in direction to the force the gases exert on the rocket (forward).
4. No external medium is needed because the interaction is internal to the rocket–exhaust system; the rocket accelerates due to this net internal exchange of momentum / the thrust arises entirely from within the system.

## Related

- [Revision notes for IB DP Physics Standard 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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