# A car travels along a straight road. Its velocity changes from +20 m s⁻¹ to −8 m s⁻¹ in a time of 7.0 s. Explain what the change in sign of the velocity indicates, and state what quantity the magnitude of the rate of change of velocity represents. Then explain why it is possible for the car's average speed over this interval to be non-zero even if its displacement over the same interval happened to be zero.

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

## Mark scheme (4 marks)

1. A change in sign of velocity indicates the car has reversed its direction of motion (is now moving in the opposite direction to its initial motion).
2. The magnitude of the rate of change of velocity represents acceleration (magnitude of acceleration).
3. Average speed is total distance travelled divided by total time, whereas average velocity is total displacement divided by total time.
4. If the displacement is zero the car has returned to its starting position, but the total distance (path length) travelled is still positive (non-zero), so the average speed is non-zero.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-car-travels-along-a-straight-1b2f5252) · Published by Druglandscape Ltd.