A cyclist travels along a straight road. For the first part of the journey the cyclist moves with a constant positive velocity, then decelerates uniformly to rest. Explain how the instantaneous acceleration of the cyclist differs from the average acceleration over the entire journey, and state the sign of each.

IB DP Physics Higher Level (2023 syllabus) — A.1 Kinematics · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Instantaneous acceleration is zero while the cyclist moves at constant velocity. During the deceleration phase the instantaneous acceleration is constant and negative. The average acceleration for the whole journey is the total change in velocity divided by the total time. It is negative, but its magnitude is smaller than the negative instantaneous acceleration because the time interval includes the constant‑velocity part where the acceleration is zero.

Examiner tips

  • Use the term ‘instantaneous acceleration’ for the momentary value; ‘average acceleration’ for the whole trip.
  • Show that the average is Δv/t and note the sign is negative.
  • Explain why the magnitude is smaller – include the zero‑acceleration period in the time denominator.

Common mistakes

  • Confusing instantaneous with average acceleration; writing the instantaneous value as non‑zero during constant speed.
  • Failing to state the sign of the average acceleration or giving the wrong magnitude comparison.
  • Omitting the definition of average acceleration (Δv/t).

Mark scheme (4 marks)

  1. Instantaneous acceleration is zero during the constant-velocity phase
  2. Instantaneous acceleration is negative (and constant) during the deceleration phase
  3. Average acceleration is defined as total change in velocity divided by total time taken
  4. Average acceleration is negative but has a smaller magnitude than the instantaneous acceleration during deceleration, because the total time includes the constant-velocity phase where no acceleration occurs

Key terms in this question

instantaneous acceleration · average acceleration

Related

More Kinematics questions

▶ Try answering this question with AI marking (free) →