A tennis ball is travelling at high speed towards a wall. The ball collides with the wall and bounces back in the opposite direction at a lower speed. Explain why the ball experiences a change in momentum during this collision, and describe what happens to the magnitude of the ball's momentum.

AQA GCSE Physics (8463) — 4.5.7 Momentum (Phys only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Momentum is the product of mass and velocity (p = mv). During the collision a force from the wall acts on the ball, changing its momentum. The ball’s velocity reverses direction, so its momentum changes direction. Because the ball leaves the wall at a lower speed than it arrived, the magnitude of its momentum decreases.

Examiner tips

  • Use the definition p = mv to show the relationship between mass, velocity and momentum. Explain that a force acting over the collision time changes momentum (impulse).
  • Mention that the direction of momentum changes when velocity reverses. State that the magnitude is lower because the speed after the collision is smaller.

Common mistakes

  • Not recognising that momentum is a vector and changes direction. Failing to mention the force/impulse that causes the change. Assuming the magnitude stays the same because mass is unchanged.

Mark scheme (4 marks)

  1. Momentum is the product of mass and velocity / momentum = mass × velocity
  2. A force acts on the ball from the wall during the collision, causing a change in momentum
  3. The ball's velocity changes direction (from towards the wall to away from the wall), so its momentum changes direction
  4. The magnitude of the ball's momentum decreases because the ball bounces back at a lower speed than it arrived (with same mass)

Key terms in this question

momentum · change in momentum

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