A cyclist travels along a flat road. At first, the cyclist accelerates. Eventually, the cyclist reaches a constant speed called terminal velocity. Explain why the cyclist reaches terminal velocity and why the cyclist's speed no longer increases after this point.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist sets off from rest along a straight, flat road. She pedals with a constant thrust force throughout her journey. Her speed gradually increases before she eventually stops accelerating and travels at a steady speed.
Model answer (4 marks)
As the cyclist’s speed increases, the air resistance (drag) acting on her also increases.
The resultant force on the cyclist therefore decreases because the opposing drag force grows.
At terminal velocity the thrust force produced by the cyclist equals the drag (and any other friction) force, so the forces are balanced.
With the resultant force zero, there is no acceleration (Newton’s First Law), so her speed remains constant.
The resultant force on the cyclist therefore decreases because the opposing drag force grows.
At terminal velocity the thrust force produced by the cyclist equals the drag (and any other friction) force, so the forces are balanced.
With the resultant force zero, there is no acceleration (Newton’s First Law), so her speed remains constant.
Examiner tips
- Use the word ‘drag’ or ‘air resistance’ to show understanding of opposing forces.
- Show the balance of forces at terminal velocity and link zero resultant force to no acceleration.
- Mention that the cyclist’s speed is constant because acceleration is zero.
- Use correct terminology: thrust, drag, resultant force, terminal velocity.
Common mistakes
- Confusing the cyclist’s speed with the speed of the road or the wind.
- Saying the cyclist stops accelerating but still continues to increase speed.
- Using incorrect terms such as ‘friction’ instead of ‘drag’ for air resistance.
Mark scheme (4 marks)
- As the cyclist's speed increases, air resistance (drag/friction) increases
- The resultant force decreases as the opposing forces increase
- At terminal velocity, the thrust force equals the drag/friction force, so the forces are balanced
- Because the resultant force is zero, there is no acceleration, so speed remains constant (Newton's First Law)
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →