A student wants to find the average diameter of a human hair. Explain why the student should measure the total width of several hairs pressed tightly together side by side, rather than measuring a single hair directly with a ruler.

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

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A single hair is too thin to be measured accurately with a ruler.

Measuring several hairs pressed tightly together gives a larger, measurable total width.

The total width is then divided by the number of hairs to obtain the width (diameter) of one hair.

This reduces the percentage uncertainty and gives a more accurate result.

Examiner tips

  • Use the word ‘because’ to link each point; show the division step; mention uncertainty reduction explicitly.

Common mistakes

  • Claiming a ruler can measure a single hair accurately; forgetting to divide the total width by the number of hairs; not explaining how uncertainty is reduced.

Mark scheme (4 marks)

  1. A single hair is too thin / too small to measure accurately with a ruler
  2. Measuring several hairs together gives a larger / measurable total width
  3. The total width is divided by the number of hairs to give the width / diameter of one hair
  4. This reduces the percentage uncertainty in the measurement / gives a more accurate result

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