Explain why a rocket moving through empty space can accelerate even though there is no medium for it to push against.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
The rocket–exhaust system is isolated, so its total momentum is conserved. Initially the system has zero momentum. When the engines fire, the hot gases are expelled rearward, giving them a negative momentum. By conservation of momentum the rocket must acquire an equal positive momentum in the forward direction. Newton’s third law also applies: the rocket exerts a rearward force on the gases and the gases exert an equal and opposite forward force on the rocket, producing thrust. Because the interaction is internal, no external medium is required; the rocket accelerates solely from the exchange of momentum within the system.
Examiner tips
- Mention conservation of momentum first, then Newton’s third law, then explain no external medium is needed.
- Use the terms ‘rocket–exhaust system’, ‘momentum’, ‘thrust’, ‘internal forces’.
- Show the direction of momentum change clearly.
- Keep the answer concise – 4 marks can be earned with 3–4 short sentences.
Mark scheme (4 marks)
- The total momentum of the rocket–exhaust system is conserved (or: the initial total momentum is zero / constant).
- 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.
- 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).
- 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
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
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