A tennis ball travelling at high speed is caught by a fielder. The fielder moves their hands backwards as they catch the ball, increasing the time over which the ball comes to rest. Explain why this technique reduces the force experienced by the fielder's hands.

Pearson Edexcel International GCSE Physics (4PH1) — 1.3 Forces, movement, shape and momentum · Explain · 4 marks · View as Markdown

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

When catching a fast-moving tennis ball, experienced fielders are taught to 'give' with the ball by pulling their hands back in the direction of the ball's motion during the catch.

Model answer (4 marks)

The ball’s momentum changes from its initial value to zero when it is stopped.

By pulling the hands back the fielder increases the time over which this change in momentum takes place.

Force is the rate of change of momentum (F = Δp ÷ Δt).

Because the same Δp occurs over a longer Δt, the force exerted on the fielder’s hands is smaller.

Examiner tips

  • Use the formula F = Δp/Δt to link time and force
  • Show that Δp is the same but Δt is increased
  • Explain that a smaller force means less impact on the hands

Common mistakes

  • Saying the ball’s speed decreases instead of its momentum
  • Confusing force with impulse
  • Ignoring that Δp is unchanged

Mark scheme (4 marks)

  1. The ball undergoes a change in momentum (from its initial momentum to zero) as it comes to rest
  2. Moving the hands backwards increases the time taken for the momentum change to occur
  3. Force equals the rate of change of momentum (force = change in momentum ÷ time)
  4. Because the same change in momentum occurs over a longer time, the force on the fielder's hands is smaller

Key terms in this question

force · time

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