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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

A rocket and its exhaust gases form a closed system, so the total momentum of the system is conserved. The exhaust gases are expelled rearwards, gaining momentum in that direction. By Newton’s third law (or conservation of momentum) the rocket gains an equal and opposite momentum in the forward direction. Because the rocket’s momentum increases over time, a net force (thrust) acts on it, causing it to accelerate forward.

Examiner tips

  • Use the term ‘closed system’ and ‘conservation of momentum’.
  • Show the direction of exhaust and rocket momentum clearly.
  • Mention Newton’s third law as the underlying principle.
  • Explain that the changing momentum gives a net force (thrust).

Common mistakes

  • Confusing the rocket’s acceleration with a need for an external medium.
  • Failing to state that the system is closed and momentum is conserved.
  • Using vague language like ‘push against air’ instead of ‘exhaust gases’.

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.

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