A tennis ball is dropped from rest and falls vertically downwards towards the ground. As the ball falls, it accelerates at first but then reaches a constant speed called terminal velocity. Explain why the ball first accelerates and then reaches terminal velocity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The ball’s weight, (W=mg), pulls it downward.
The air resistance, (F_{ ext{drag}}), acts upward and opposes the motion.
At the start the ball is at rest, so (F_{ ext{drag}}approx0) and (W>F_{ ext{drag}}); the net force is downward and the ball accelerates.
As the speed increases, (F_{ ext{drag}}) increases (approximately (F_{ ext{drag}}propto v^{2})).
When (F_{ ext{drag}}) becomes equal to (W), the net force is zero and the ball no longer accelerates – it moves at the constant terminal velocity.
The air resistance, (F_{ ext{drag}}), acts upward and opposes the motion.
At the start the ball is at rest, so (F_{ ext{drag}}approx0) and (W>F_{ ext{drag}}); the net force is downward and the ball accelerates.
As the speed increases, (F_{ ext{drag}}) increases (approximately (F_{ ext{drag}}propto v^{2})).
When (F_{ ext{drag}}) becomes equal to (W), the net force is zero and the ball no longer accelerates – it moves at the constant terminal velocity.
Examiner tips
- Use the terms weight, air resistance and terminal velocity. Show the change of net force from downward to zero. Keep the explanation concise and directly linked to the four scheme points.
Common mistakes
- Confusing air resistance with friction. Saying the ball keeps accelerating forever. Using vague phrases like ‘the ball slows down’ instead of ‘net force becomes zero’.
Mark scheme (4 marks)
- Weight (gravitational force) acts downward on the ball
- Air resistance (drag) acts upward on the ball, opposing its motion
- Initially weight is greater than air resistance, so there is a resultant force downward, causing acceleration
- As speed increases, air resistance increases until it equals weight, so the resultant force is zero and the ball moves at constant (terminal) velocity
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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