A sprinter runs a 100 m race along a straight track. She reaches the finish line and then slows down, stopping completely 40 m beyond the finish line. Explain the difference between the sprinter's average speed and her average velocity for the entire journey from start to final stopping point, and state which quantity is larger.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A sprinter completes a 100 m race and continues running past the finish line before stopping 40 m beyond it. Her total journey from start to stop covers 140 m along a straight track.
Model answer (4 marks)
Average speed is the total distance travelled divided by the time taken. In this case the sprinter covers 140 m, so her average speed is 140 m ÷ t.
Average velocity is the displacement (the straight‑line distance from start to finish) divided by the same time. The displacement is 100 m, so her average velocity is 100 m ÷ t.
Because the displacement (100 m) is less than the total distance travelled (140 m), the average speed is larger than the average velocity for the whole journey.
Average velocity is the displacement (the straight‑line distance from start to finish) divided by the same time. The displacement is 100 m, so her average velocity is 100 m ÷ t.
Because the displacement (100 m) is less than the total distance travelled (140 m), the average speed is larger than the average velocity for the whole journey.
Examiner tips
- Use the definitions: speed = distance ÷ time, velocity = displacement ÷ time. Show the two numerators (140 m vs 100 m) and note they share the same denominator. State clearly that speed > velocity because 140 m > 100 m.
Common mistakes
- Confusing displacement with distance, giving 140 m for both. Failing to recognise that the time is the same for both calculations. Using the wrong sign for velocity (e.g., 100 m in the wrong direction).
Mark scheme (4 marks)
- Speed is the distance travelled divided by time (or speed uses total distance of 140 m)
- Velocity is displacement divided by time (or velocity uses displacement of 100 m in the direction of travel)
- Displacement (100 m) is less than total distance travelled (140 m)
- Average speed is larger than average velocity because it is calculated using the greater value (distance) and the same time
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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