A sprinter runs 100 m along a straight track in 12.5 s, then immediately turns around and jogs back 40 m towards the start line before stopping. Describe the difference between the sprinter's total distance travelled and their displacement at the point they stop, and explain why their average speed and average velocity for the whole journey are different from each other.

WJEC A-Level Physics (Wales) — 1.1 Basic physics quantities and motion · Describe and Explain · 5 marks · View as Markdown

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

A sprinter completes a 100 m race in 12.5 s, then turns and jogs back 40 m along the same straight track before stopping.

Model answer (5 marks)

Total distance travelled is 140 m (100 m + 40 m). The sprinter’s displacement is 60 m, in the original direction of travel, because the final position is 60 m from the start line. Distance is a scalar quantity – it has only magnitude – whereas displacement is a vector – it has both magnitude and direction. Average speed is calculated from total distance: 140 m ÷ 12.5 s = 11.2 m s⁻¹. Average velocity is calculated from displacement: 60 m ÷ 12.5 s = 4.8 m s⁻¹. Therefore the average speed and average velocity are different because speed uses total distance while velocity uses displacement.

Examiner tips

  • Use the exact values 140 m and 60 m; show the division for speed and velocity; remember to state that speed is scalar and velocity is vector.
  • Explain why the two averages differ – speed uses total distance, velocity uses displacement.
  • Use the correct units (m s⁻¹).

Common mistakes

  • Confusing distance with displacement; giving the same value for both.
  • Calculating average velocity with total distance instead of displacement.
  • Omitting the direction in the displacement description.

Mark scheme (5 marks)

  1. Total distance travelled is 140 m (100 m + 40 m)
  2. Displacement is 60 m (in the original direction of travel / from start to final position)
  3. Displacement is a vector quantity / has both magnitude and direction, whereas distance is a scalar (magnitude only)
  4. Average speed uses total distance (140 m) in its calculation
  5. Average velocity uses displacement (60 m) instead of total distance, so it gives a smaller / different value

Key terms in this question

distance · displacement · average speed · average velocity

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