AIR RESISTANCE AND TERMINAL VELOCITY
3 marks · EasyA cyclist is moving quickly down a hill and eventually reaches a speed where her speed does not increase any further, even though she is still going downhill.
Explain why the cyclist does not keep speeding up. In your answer, refer to the forces acting on the cyclist.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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The cyclist reaches a speed where the force of gravity down the hill is balanced by air resistance (and friction) acting against her. Since the forces are balanced, there is no resultant force, so the cyclist no longer accelerates and moves at a constant (terminal) speed.
Also accept: At terminal velocity, resisting forces match the driving force so no further acceleration
Common mistakes: Not mentioning balance of forces; Only saying air resistance exists
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Marking points
B1 · 1 mark · Mentions the driving force (gravity down hill) and opposing force (air resistance or friction)
B1 · 1 mark · States or clearly implies the forces are balanced at the constant speed
B1 · 1 mark · States that no resultant force means no further acceleration (moves at constant speed or terminal velocity)
Worked solution
As the cyclist moves faster, air resistance (and friction) increase until they balance the force of gravity pulling her downhill. When these forces are balanced, there is no resultant force, so she no longer accelerates and continues at a constant speed. This speed is called terminal velocity.