A cyclist rides along a flat road at a constant speed. The cyclist does work against air resistance and friction. Explain why the cyclist must continuously do work to maintain a constant speed, and describe what happens to the energy that is transferred to the surroundings.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist travels at a steady speed along a flat road. The cyclist's muscles provide a driving force, but resistive forces act against the motion.
Model answer (5 marks)
A resistive force (air resistance and rolling friction) opposes the cyclist’s motion.
The cyclist’s muscles must supply a driving force equal to this resistive force so that the net force is zero and the speed remains constant.
Because the driving force does work against the resistive force, mechanical energy is transferred from the cyclist to the surroundings.
The transferred energy is dissipated as heat (thermal energy) in the air, tyres and rider’s body.
Thus the total energy is conserved – it is not created or destroyed, only transferred and spread out as heat, making it unavailable for useful work.
The cyclist’s muscles must supply a driving force equal to this resistive force so that the net force is zero and the speed remains constant.
Because the driving force does work against the resistive force, mechanical energy is transferred from the cyclist to the surroundings.
The transferred energy is dissipated as heat (thermal energy) in the air, tyres and rider’s body.
Thus the total energy is conserved – it is not created or destroyed, only transferred and spread out as heat, making it unavailable for useful work.
Examiner tips
- Show the balance of forces at constant speed; mention work done against resistive forces; state energy becomes heat; note conservation of energy; keep answer concise and use correct terminology.
Common mistakes
- Confusing the cyclist’s speed with acceleration; not recognising that work is done against resistive forces; claiming energy is destroyed instead of transferred as heat.
Mark scheme (5 marks)
- A resistive force (air resistance / friction) opposes the motion of the cyclist
- Work is done against the resistive force, so energy is transferred from the cyclist to the surroundings
- At constant speed, the driving force equals the resistive force (forces are balanced)
- The energy transferred to the surroundings is dissipated as heat (thermal energy)
- The total energy is conserved — energy is not created or destroyed, only transferred; the system is less useful because the dissipated energy is spread out and cannot be recovered to do useful work
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →