# A student measures the length of a short metal rod using a ruler and records a single measurement of 8 mm. Explain why this method produces a result with a high percentage uncertainty, and state one change the student could make to reduce this percentage uncertainty.

> Cambridge International IGCSE Physics (0625) — 1.1 Physical quantities and measurement techniques · Explain · 4 marks

## Mark scheme (4 marks)

1. A small measurement has a high percentage uncertainty because the uncertainty is a large fraction / proportion of the measured value
2. The absolute uncertainty of a ruler reading is fixed (e.g. ±0.5 mm or ±1 mm), so when the measured value is small, the percentage uncertainty is large
3. Use a micrometer screw gauge instead of a ruler to measure the rod
4. A micrometer screw gauge measures to 0.01 mm, so the absolute uncertainty is much smaller relative to the measurement, giving a lower percentage uncertainty

## Key terms

- [percentage uncertainty](https://www.gradenine.co.uk/glossary/percentage-uncertainty)

## 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-student-measures-the-length-of-30331f73) · Published by Druglandscape Ltd.