A rubber ball is thrown vertically upwards and then falls back down. As the ball rises, the resultant force on the ball acts downwards. Explain why the ball eventually stops rising and then falls back down, referring to the resultant force and the direction of the ball's motion.

Cambridge International IGCSE Physics (0625) — 1.5 Forces · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The ball is acted on by its weight, which is a downward force that is always present. While the ball is rising, this downward resultant force is opposite to the direction of motion, so it decelerates the ball. As the upward speed decreases, the ball eventually reaches a point where its speed is zero – the highest point. At this instant the resultant force is still downward, so the ball begins to accelerate in that direction and falls back down.

Examiner tips

  • Use the word ‘weight’ or ‘gravitational force’ to show understanding of the force involved.
  • Explain that the resultant force is opposite to the motion while rising, causing deceleration.
  • Mention that the speed becomes zero at the top before the ball reverses direction.
  • Show that the same downward force then causes acceleration downwards.

Mark scheme (4 marks)

  1. Weight / gravitational force acts downwards on the ball throughout its motion
  2. The resultant force acts downwards (opposite to the direction of motion as the ball rises), so the ball decelerates
  3. The ball's upward speed decreases to zero / the ball momentarily stops at the highest point
  4. The resultant force (weight) still acts downwards, so the ball accelerates downwards / falls back down

Key terms in this question

resultant force

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