A tennis ball is dropped from rest and falls vertically downward. It hits the ground and bounces back upward. Explain what happens to the forces acting on the tennis ball from the moment it is dropped until it reaches its highest point after bouncing.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
During the fall the only force on the ball is its weight, so it accelerates downward.
When it strikes the ground a large upward normal force acts on it, larger than its weight.
After leaving the ground the only force again is its weight, which now acts opposite to the motion and decelerates the ball.
At the highest point the ball is momentarily at rest but its weight is still the only force acting.
When it strikes the ground a large upward normal force acts on it, larger than its weight.
After leaving the ground the only force again is its weight, which now acts opposite to the motion and decelerates the ball.
At the highest point the ball is momentarily at rest but its weight is still the only force acting.
Examiner tips
- Use the word ‘only’ to show the single force during the fall and at the top. Explain that the normal force exceeds weight on impact. Show the change in direction of the resultant force when the ball leaves the ground. Mention that the ball is momentarily stationary but weight remains unbalanced.
Common mistakes
- Saying the ball is in free fall after the bounce. Forgetting that the normal force is greater than weight on impact. Claiming that the ball is weightless at the top.
Mark scheme (5 marks)
- During the fall, the only force acting (before contact) is weight / gravitational force downward
- The ball accelerates downward during the fall because there is a resultant force downward (weight is unbalanced / no air resistance mentioned OR air resistance is less than weight)
- On contact with the ground, a large contact / normal / reaction force acts upward on the ball (greater than weight)
- After leaving the ground, weight acts downward and the ball decelerates as it moves upward (resultant force is downward, opposing motion)
- At the highest point after bouncing, the ball is momentarily stationary but weight still acts downward (forces are not balanced / weight is still the only force)
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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