A cyclist travels at a constant speed along a flat road, then stops pedalling and gradually slows to rest. State what is meant by the term 'deceleration' and describe how the speed–time graph of the cyclist's journey would differ between the constant-speed phase and the slowing phase.

Cambridge International IGCSE Physics (0625) — 1.2 Motion · State and Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Deceleration is the negative acceleration, i.e. the rate of decrease of speed.

During the constant‑speed phase the speed–time graph is a horizontal line (gradient = 0).

During the slowing phase the graph slopes downward – a negative gradient. The magnitude of the gradient equals the deceleration; a steeper negative slope means a greater deceleration.

Examiner tips

  • Use the term ‘deceleration’ as negative acceleration; link it to the rate of decrease of speed.
  • State that the constant‑speed section is a horizontal line (zero gradient).
  • State that the slowing section is a downward sloping line (negative gradient).
  • Mention that the steepness of the slope gives the magnitude of the deceleration.

Common mistakes

  • Writing ‘deceleration is a positive acceleration’ or ‘speed decreases’ without mentioning negative acceleration.
  • Describing the constant‑speed section as a sloping line instead of horizontal.
  • Forgetting to note that the slope of the slowing section is negative or that its magnitude gives the deceleration.

Mark scheme (4 marks)

  1. Deceleration is negative acceleration / the rate of decrease of speed (or velocity)
  2. During the constant-speed phase the line is horizontal (gradient is zero)
  3. During the slowing phase the line slopes downward (has a negative gradient)
  4. The gradient of the slowing section represents the (magnitude of the) deceleration / a steeper negative gradient means greater deceleration

Key terms in this question

deceleration · speed–time graph · constant speed

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