A bicycle uses a gear system consisting of a large front chainring connected by a chain to a small rear sprocket. When the cyclist pedals, the rear wheel spins faster than the pedals. Explain how the gear system achieves this and describe one disadvantage of using this gear arrangement when cycling uphill.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cyclists use different gear combinations to suit different riding conditions. A large front chainring paired with a small rear sprocket is often called a 'high gear'.
Model answer (4 marks)
A large front chainring drives a small rear sprocket by the chain. Because the sprocket has a smaller radius, it must rotate more times to cover the same distance of chain, so the rear wheel turns faster than the pedals.
The rear wheel is fixed to the sprocket, so its angular speed is the same as the sprocket’s. Thus the wheel spins faster than the pedals.
However, the torque required to turn the large chainring is greater than that needed for the small sprocket. The cyclist must apply a larger force on the pedals to overcome this, which means more effort is required.
When climbing, this higher required torque makes it difficult for the cyclist to generate enough force, so the bike is hard to pedal uphill.
The rear wheel is fixed to the sprocket, so its angular speed is the same as the sprocket’s. Thus the wheel spins faster than the pedals.
However, the torque required to turn the large chainring is greater than that needed for the small sprocket. The cyclist must apply a larger force on the pedals to overcome this, which means more effort is required.
When climbing, this higher required torque makes it difficult for the cyclist to generate enough force, so the bike is hard to pedal uphill.
Examiner tips
- Use the word ‘torque’ or ‘moment’ when explaining the force requirement
- Show the chainring–sprocket relationship clearly
- Mention the wheel speed relative to the pedals
Common mistakes
- Confusing the direction of rotation or saying the wheel turns slower
- Forgetting to explain why more force is needed
- Not linking the disadvantage to uphill climbing
Mark scheme (4 marks)
- The large gear (chainring) turns the small gear (sprocket) — the small gear rotates more times for each rotation of the large gear
- The rear wheel is attached to the sprocket and so also spins faster than the pedals / chainring
- A greater force (moment/torque) is needed to turn the larger chainring relative to the small sprocket, so the cyclist must push harder on the pedals
- This makes it harder to pedal uphill because the cyclist cannot generate enough force / it is harder to overcome the increased resistance when climbing
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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