A cyclist rides along a flat road at a constant speed. Explain why the cyclist must continue to pedal in order to maintain this constant speed, and describe what would happen to the motion of the cyclist if they stopped pedalling.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist is travelling along a flat, straight road at a steady speed. The cyclist notices that whenever they stop pedalling, they gradually slow down and eventually stop.
Model answer (4 marks)
At constant speed the cyclist’s thrust from pedalling balances the resistive forces (friction and air resistance), giving a zero resultant force.
If the cyclist stops pedalling the thrust disappears, so the resistive forces are unopposed. A resultant force then acts opposite to the motion, causing the cyclist to decelerate, slow down and eventually come to rest.
If the cyclist stops pedalling the thrust disappears, so the resistive forces are unopposed. A resultant force then acts opposite to the motion, causing the cyclist to decelerate, slow down and eventually come to rest.
Examiner tips
- Use the word ‘balance’ to link thrust and resistance; mention zero resultant force. Explain that removing thrust leaves only resistive forces, giving a non‑zero resultant. Show the direction of the resultant and its effect (deceleration).
Common mistakes
- Failing to state that thrust equals resistance at constant speed. Forgetting that the resultant force is opposite to motion. Using vague terms like ‘slow down’ without linking to forces.
Mark scheme (4 marks)
- At constant speed, the thrust force from pedalling equals/balances the resistive forces (friction and/or air resistance/drag), giving a zero resultant force.
- When the cyclist stops pedalling, the thrust force is removed/becomes zero.
- There is now a resultant force acting on the cyclist in the direction opposite to motion (due to friction and/or air resistance).
- This resultant force causes the cyclist to decelerate (slow down and eventually stop).
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Forces and their effects questions
- A heavy wooden crate is pushed across a rough concrete floor at a constant veloc…
- A steel ball bearing is held near a strong bar magnet but is not touching it. Ex…
- A car accelerates from rest along a straight, level road. The engine provides a …
- A skydiver jumps from an aircraft and falls towards the ground. Explain why the …