# A sprinter runs 100 m along a straight track in 12.5 s and then immediately turns around and jogs 40 m back towards the starting line in 20 s. Explain why the sprinter's average speed for the whole journey is different from the magnitude of the sprinter's average velocity for the whole journey.

> Cambridge International IGCSE Physics (0625) — 1.2 Motion · Explain · 4 marks

## Mark scheme (4 marks)

1. Average speed uses total distance travelled (140 m)
2. Average velocity uses displacement (60 m), which is the overall change in position / distance from start to finish point
3. Speed is a scalar (distance ÷ time) so direction is not considered, whereas velocity is a vector (displacement ÷ time) so direction is considered
4. Because total distance (140 m) is greater than the magnitude of displacement (60 m), and the total time is the same for both calculations, average speed is greater than the magnitude of average velocity

## Key terms

- [average speed](https://www.gradenine.co.uk/glossary/average-speed)
- [average velocity](https://www.gradenine.co.uk/glossary/average-velocity)

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-sprinter-runs-100-m-along-3e500fec) · Published by Druglandscape Ltd.