A large cargo ship and a small speedboat are both travelling at the same velocity across a harbour. Explain why the cargo ship is much harder to stop than the speedboat.

Cambridge International IGCSE Physics (0625) — 1.6 Momentum · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Momentum is the product of mass and velocity.
2. The cargo ship has a much greater mass than the speedboat.
3. Because the velocity is the same, the cargo ship therefore has a much greater momentum than the speedboat.
4. A greater force (or a longer time) is required to change (reduce to zero) the cargo ship’s momentum, so it is harder to stop.

Examiner tips

  • Use the definition of momentum (m×v) early. State the mass difference explicitly. Show the logical chain: same v → greater m → greater p. Mention force or time needed to change momentum.
  • common_mistakes
  • :
  • Confusing mass with weight. Forgetting that the velocity is the same for both. Saying the ship is heavier without linking it to momentum.

Mark scheme (4 marks)

  1. Momentum is the product of mass and velocity
  2. The cargo ship has a much greater mass than the speedboat
  3. Because velocity is the same, the cargo ship has much greater momentum than the speedboat
  4. A greater force (or longer time) is needed to change (reduce to zero) the momentum of the cargo ship

Key terms in this question

velocity

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