A cyclist rides along a straight, flat road. She accelerates from rest, travels at a constant speed, and then decelerates to a stop. Describe how the distance travelled by the cyclist changes over time during each of the three stages of her journey, and explain what the motion tells us about the forces acting on the cyclist during the constant speed stage.

OCR A-Level Physics B: Advancing Physics (H557) — 3.1 Motion · Describe · 5 marks · View as Markdown

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

A cyclist starts from rest at a set of traffic lights. She accelerates steadily, then maintains a constant speed for several seconds, before braking to a stop at the next set of traffic lights.

Model answer (5 marks)

During the acceleration stage the distance–time graph is curved, with the gradient increasing as time goes on – the cyclist covers more distance in each successive time interval.
During the constant‑speed stage the graph is a straight line; the gradient is constant, meaning equal distances are covered in equal times.
During the deceleration stage the graph is still curved but the gradient decreases – the cyclist covers less distance in each successive time interval.
At constant speed the resultant force on the cyclist is zero.
At constant speed the forward driving force exerted by the cyclist balances the resistive forces such as friction, air resistance and other drag forces.

Examiner tips

  • Use the distance–time graph to show the changing gradient for each stage; mention ‘curved’ and ‘straight line’.
  • State that at constant speed the net force is zero and the cyclist’s forward force equals resistive forces.
  • Include the word ‘steady’ or ‘uniform’ for the constant‑speed stage.

Common mistakes

  • Confusing acceleration with deceleration, or describing the constant‑speed stage as still accelerating.
  • Forgetting to mention that the net force is zero at constant speed.
  • Using vague terms like ‘slowly’ without linking to the gradient of the distance–time graph.

Mark scheme (5 marks)

  1. During acceleration, the distance travelled increases at a faster and faster rate (distance–time relationship is curved/getting steeper).
  2. During constant speed, distance increases at a steady/uniform rate (equal distance covered in equal time intervals).
  3. During deceleration, the distance travelled still increases but at a slower and slower rate (gradient decreases / curves and flattens).
  4. At constant speed the resultant force on the cyclist is zero.
  5. At constant speed the driving/forward force from the cyclist equals the resistive forces (such as friction/air resistance/drag) acting on her.

Key terms in this question

constant speed

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