A rocket is launched from rest into space. Once in deep space, far from any gravitational fields, the rocket engines are switched off. The rocket continues to move at a constant velocity. A small thruster then fires briefly, pushing the rocket sideways. Explain, using Newton's laws of motion and the concept of momentum, why the rocket continues at constant velocity when the engines are off, and what happens to the rocket's momentum when the thruster fires.

OCR A-Level Physics B: Advancing Physics (H557) — 3.5 Newton's laws and momentum · Explain · 5 marks · View as Markdown

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

A rocket travelling in deep space has its main engines switched off. It continues moving at a constant velocity. A small thruster then fires briefly, exerting a force on the rocket for a short time.

Model answer (5 marks)

When the engines are off there is no external force on the rocket, so the resultant force is zero. By Newton’s first law the rocket therefore continues at a constant velocity (or remains at rest). When the thruster fires it produces a force on the rocket, giving a non‑zero resultant force. By Newton’s second law this force causes an acceleration, changing the rocket’s velocity. Because momentum p=mv, the change in velocity means the rocket’s momentum changes in the direction of the thrust.

Examiner tips

  • State the absence of forces first, then cite Newton 1; link the thruster force to Newton 2 and the change in velocity; finish with the momentum equation.
  • Use the exact wording ‘resultant force’, ‘Newton’s first law’, ‘Newton’s second law’, and ‘momentum = mass × velocity’.
  • Show the logical sequence: no force → constant velocity; force → acceleration → change in velocity → change in momentum.

Common mistakes

  • Claiming the rocket stops when the engines are off; ignoring the zero‑force condition.
  • Confusing the thruster’s force with the rocket’s internal forces; not linking to momentum change.
  • Using vague terms like ‘speed up’ instead of ‘accelerate’ or ‘change velocity’.

Mark scheme (5 marks)

  1. When the engines are off, there are no unbalanced (resultant) forces acting on the rocket in deep space
  2. By Newton's first law, an object continues at constant velocity (or remains at rest) unless acted on by a resultant force
  3. When the thruster fires, it exerts a resultant force on the rocket
  4. By Newton's second law (or the relationship between force, mass and acceleration), this force causes the rocket to accelerate / change velocity
  5. The rocket's momentum changes because momentum = mass × velocity, and the force from the thruster causes a change in velocity (and therefore a change in momentum)

Key terms in this question

momentum · constant velocity

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