A tennis ball is held at rest against a wall by a player's hand. Explain, using Newton's laws, what happens to the forces on the ball when the player removes their hand and allows the ball to fall freely.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A tennis ball is pressed against a smooth vertical wall. The player holds the ball in place with their hand, then lets go. The ball then falls straight down.
Model answer (4 marks)
Whilst the ball is held against the wall the forces on it are balanced – the normal force from the wall and the force from the hand act horizontally, while the weight acts vertically; the resultant force is zero. By Newton’s First Law the ball therefore remains at rest. When the hand is removed the only force acting on the ball is its weight, which acts vertically downwards. This gives a non‑zero resultant force, so by Newton’s Second Law the ball accelerates downwards.
Examiner tips
- State the balance of forces while the ball is held; mention the normal force and weight. Use the phrase "resultant force is zero" to show understanding of equilibrium. Explain that removal of the hand leaves only weight, giving a non‑zero resultant. Link the non‑zero resultant to acceleration via Newton’s Second Law.
Common mistakes
- Confusing the normal force with the weight; forgetting that the normal force is horizontal. Failing to mention that the resultant force becomes non‑zero after the hand is removed. Using vague terms like "the ball falls" without linking to Newton’s laws.
Mark scheme (4 marks)
- Whilst held stationary, the forces on the ball are balanced / the resultant force is zero
- Newton's First Law: because the resultant force is zero, the ball remains stationary
- When the hand is removed, the only force acting is weight / gravity acting vertically downwards, so there is now a resultant force
- Newton's Second Law / First Law: the resultant force (weight) causes the ball to accelerate downwards
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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