A cyclist changes to a larger gear on their bicycle. Describe and explain how this affects the force and speed of the rear wheel compared to using a smaller gear.

AQA GCSE Physics (8463) — 4.5.4 Moments, levers and gears (Phys only) · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A bicycle uses a system of gears connected by a chain. The pedal gear drives a rear wheel gear.

Model answer (4 marks)

A larger gear turns more slowly than a smaller gear when driven by the same input gear.

The larger gear transmits a greater force to the rear wheel.

Because the force acts at a greater perpendicular distance from the axle, the moment (torque) on the rear wheel is larger.

A larger moment means the rear wheel turns more slowly, so the cyclist’s speed is reduced while the force on the wheel is increased.

Examiner tips

  • Use the word ‘turns more slowly’ to show speed change. Mention that the force is greater and acts farther from the axle. Explain that a larger moment gives more turning force, reducing speed but increasing force.
  • common_mistakes
  • :
  • Confusing speed of the wheel with speed of the cyclist; the wheel speed is lower. Forgetting to link the larger radius to a larger moment. Using vague terms like ‘more power’ instead of ‘greater force’ and ‘larger torque’.

Mark scheme (4 marks)

  1. A larger gear turns more slowly than a smaller gear (when driven by the same input gear)
  2. A larger gear produces a greater force / more force is transmitted to the rear wheel
  3. The force acts at a greater perpendicular distance from the pivot / axle of the larger gear
  4. This gives a larger moment on the larger gear, so more turning force is produced

Key terms in this question

gear

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