A ball is thrown horizontally from the top of a cliff and lands on the ground below. Explain why the ball's speed increases from the moment it is released until it reaches the ground, and state the name given to the constant speed that a falling object can reach when passing through air.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A ball is thrown horizontally from the top of a cliff. Air resistance acts on the ball throughout its fall.
Model answer (4 marks)
The ball is acted on by its weight, which pulls it downward.
The only other force is air resistance, which is upward but smaller than the weight.
Because the resultant force is downward, the ball accelerates and its speed increases.
As the speed rises, the air resistance grows, but it never exceeds the weight until the ball reaches the ground.
When the weight equals the air resistance the resultant force is zero, so the ball reaches a constant speed called terminal velocity.
The only other force is air resistance, which is upward but smaller than the weight.
Because the resultant force is downward, the ball accelerates and its speed increases.
As the speed rises, the air resistance grows, but it never exceeds the weight until the ball reaches the ground.
When the weight equals the air resistance the resultant force is zero, so the ball reaches a constant speed called terminal velocity.
Examiner tips
- Use the word ‘weight’ and ‘air resistance’ to show you know the forces. Show the direction of the resultant force and link it to acceleration. Explain how the increase in speed changes the air resistance. State the term ‘terminal velocity’ clearly.
- common_mistakes
- :
- Confusing the direction of the forces or saying the air resistance is greater than the weight. Forgetting to mention that the resultant force is downward. Using the wrong term for the constant speed (e.g., ‘steady speed’ instead of ‘terminal velocity’.
Mark scheme (4 marks)
- The weight (gravitational force) acts downward on the ball / gravity accelerates the ball downward
- The resultant force is downward (because weight is greater than air resistance), causing acceleration / increasing speed
- As speed increases, air resistance increases (but remains less than weight throughout the fall to the ground)
- Terminal velocity — the constant speed reached when weight equals air resistance (zero resultant force / zero acceleration)
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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