A train travels from one city to another city and then returns to the starting city. Explain why the train's displacement at the end of the journey is zero, even though its average speed during the journey is not zero.

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

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

Model answer (4 marks)

Displacement is the change in position from start to finish. The train ends at the same point it started, so its displacement is zero.
Average speed is the total distance divided by time. The train travels a non‑zero distance on the way out and back, so its average speed is not zero.

Examiner tips

  • Use the definition of displacement as a vector change in position
  • Explain that the return journey cancels the outward displacement
  • Mention that average speed uses total distance, a scalar, not displacement
  • Keep the answer concise and to the point

Common mistakes

  • Confusing displacement with distance
  • Saying the average speed is zero
  • Not recognising that displacement is a vector and distance a scalar

Mark scheme (4 marks)

  1. Displacement measures the overall change in position / distance in a given direction from start to finish
  2. The train returns to its starting position, so its final position equals its initial position
  3. Average speed is calculated using total distance travelled (not displacement), which is non-zero
  4. Distance is a scalar quantity so it does not cancel when the direction reverses, whereas displacement is a vector so the return journey cancels the outward journey

Key terms in this question

displacement · average speed

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