A tennis ball is travelling towards a wall at 8 m/s. It bounces off the wall and travels back in the opposite direction at 8 m/s. State what happens to the speed, the velocity, and the momentum of the tennis ball as a result of the collision with the wall. Justify your answer for momentum.

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

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

Model answer (4 marks)

Speed remains unchanged at 8 m/s.
Velocity changes direction, so it is different.
Momentum changes.
Momentum changes because it is a vector; a change in direction of velocity changes the direction of the momentum vector, so the momentum is different.

Examiner tips

  • Use the word ‘remains’ for speed, ‘changes’ for velocity and momentum.
  • Explain that momentum depends on velocity, not just magnitude.
  • Show the vector nature of momentum in the justification.

Common mistakes

  • Saying speed changes or leaving out the word ‘remains’. Confusing velocity with speed. Failing to mention that momentum is a vector and depends on direction.

Mark scheme (4 marks)

  1. The speed remains unchanged / stays the same (at 8 m/s)
  2. The velocity changes / is different (because direction has reversed / changed)
  3. The momentum changes / is different after the collision
  4. Momentum changes because momentum is a vector quantity, so a change in direction means a change in momentum / momentum depends on velocity (which has changed)

Key terms in this question

speed · velocity · momentum

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