# A student measures the length of a piece of string using a ruler. Another student then measures the speed of a toy car as it rolls across a floor, timing it with a stopwatch over a measured distance. Explain the difference between a scalar quantity and a vector quantity, and for each measurement taken by the students — length, distance, time and speed — state whether it is a scalar or a vector quantity, justifying your answer.

> OCR A-Level Physics A (H556) — 2.1 Physical quantities and units · Explain · 5 marks

> In physics, all measured quantities can be classified as either scalar or vector quantities. Understanding this distinction is fundamental to describing motion correctly.

## Mark scheme (5 marks)

1. A scalar quantity has magnitude only (and no direction).
2. A vector quantity has both magnitude and direction.
3. Length and distance are scalar quantities because they only measure how far an object has moved/how long something is, with no direction needed.
4. Time is a scalar quantity because it has magnitude (an amount) but no direction.
5. Speed is a scalar quantity because it describes how fast an object moves but does not include direction; velocity (not speed) is the vector equivalent as it gives speed in a given direction.

## Key terms

- [scalar](https://www.gradenine.co.uk/glossary/scalar)
- [vector](https://www.gradenine.co.uk/glossary/vector)
- [distance](https://www.gradenine.co.uk/glossary/distance)
- [speed](https://www.gradenine.co.uk/glossary/speed)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](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-measures-the-length-of-8fc2b8ea) · Published by Druglandscape Ltd.