# A student is timing a remote-controlled car as it moves along a straight road. The car travels 30 m in one direction in 10 s, then immediately reverses and travels 10 m back along the same straight road in 5 s. Explain the difference between the car's average speed for the whole journey and its average velocity for the whole journey, and describe how each quantity is calculated using the car's motion.

> WJEC A-Level Physics (Wales) — 1.1 Basic physics quantities and motion · Explain · 5 marks

> A remote-controlled car travels 30 m along a straight road in 10 s, then immediately reverses and travels 10 m back along the same straight road in 5 s.

## Mark scheme (5 marks)

1. Average speed uses total distance travelled (40 m), not displacement
2. Average speed = total distance ÷ total time = 40 m ÷ 15 s (≈ 2.7 m/s)
3. Displacement is 20 m (in the original direction of travel) because the car ends up 20 m from its start
4. Average velocity = displacement ÷ total time = 20 m ÷ 15 s (≈ 1.3 m/s)
5. Velocity is a vector quantity so it has both magnitude and direction / speed is a scalar and has no direction

## 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 WJEC A-Level Physics (Wales)](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-student-is-timing-a-remote-controlled-2cef5981) · Published by Druglandscape Ltd.