A tennis ball is dropped from rest and falls vertically downwards towards the ground. Explain why the ball accelerates as it falls, and state what happens to the acceleration as the ball gains speed.

Edexcel GCSE Physics (1PH0) — 2.2 Forces and motion · Explain · 4 marks · View as Markdown

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

A tennis ball is released from rest at the top of a tall building and falls towards the ground.

Model answer (4 marks)

The ball’s weight (gravitational force) pulls it downwards.
Air resistance acts upward, opposing the motion.
Because the weight is larger than the air resistance, a net downward force remains.
This net force causes the ball to accelerate.
As the ball speeds up, the air resistance grows, reducing the net force and so the acceleration decreases.

Examiner tips

  • Use the word ‘net force’ to link weight and air resistance. Show the direction of each force. Explain the change in acceleration in terms of the changing net force.
  • common_mistakes
  • :
  • Confusing weight with mass. Forgetting that air resistance increases with speed. Saying the acceleration stays the same or increases.

Mark scheme (4 marks)

  1. Weight (gravitational force) acts downwards on the ball
  2. Air resistance acts upwards on the ball (opposing motion)
  3. Weight is greater than air resistance, so there is a resultant force downwards causing acceleration
  4. As speed increases, air resistance increases, so the resultant force decreases and the acceleration decreases

Key terms in this question

acceleration

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