A cyclist rides along a flat road at a constant speed. The cyclist's legs apply a force to the pedals, but the bicycle does not accelerate. Explain why the kinetic energy of the cyclist and bicycle does not increase, and describe what happens to the energy transferred by the cyclist's legs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist pedals continuously along a flat road at a steady speed. Despite the constant effort from the cyclist, the speed does not change.
Model answer (5 marks)
The cyclist’s legs do work on the pedals, so energy is transferred into the bicycle‑cyclist system.
1. The force from the legs acts over the distance the pedals turn, giving the system kinetic energy.
2. On a flat road the only significant resistive forces are friction in the bearings and air resistance.
3. Because the cyclist is travelling at a constant speed, the driving force equals the resistive forces; the net force is zero.
4. With no net force the kinetic energy of the cyclist and bicycle does not change – it remains constant.
5. The energy supplied by the cyclist is therefore not stored as kinetic energy but is dissipated as heat in the frictional contacts and in the air (a waste energy transfer).
1. The force from the legs acts over the distance the pedals turn, giving the system kinetic energy.
2. On a flat road the only significant resistive forces are friction in the bearings and air resistance.
3. Because the cyclist is travelling at a constant speed, the driving force equals the resistive forces; the net force is zero.
4. With no net force the kinetic energy of the cyclist and bicycle does not change – it remains constant.
5. The energy supplied by the cyclist is therefore not stored as kinetic energy but is dissipated as heat in the frictional contacts and in the air (a waste energy transfer).
Examiner tips
- Show the work done by the legs first, then explain the balance of forces, and finish with the energy dissipation. Use the words ‘work’, ‘force’, ‘resistive’, ‘heat’ and ‘conservation of energy’.
- Remember the 5‑mark structure: 1‑2 points for work, 1 for friction, 1 for force balance, 1 for heat, 1 for conservation.
Common mistakes
- Saying the kinetic energy increases; forgetting that constant speed means no acceleration.
- Mixing up the direction of the frictional force or not recognising it as a resistive force.
- Claiming the energy disappears instead of being converted to heat.
Mark scheme (5 marks)
- Work is done by the cyclist's legs / a force acts over a distance, so energy is transferred into the system
- Friction / air resistance acts against the motion of the bicycle
- The driving force from the cyclist equals / is balanced by the resistive forces, so there is no net force
- The energy transferred by the cyclist is a waste energy transfer — it is dissipated as thermal energy (heat) due to friction / air resistance
- Energy is not created or destroyed — conservation of energy means the total energy input equals the total energy dissipated / the kinetic energy store stays constant because all input energy becomes waste thermal energy
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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