A student wants to find the density of a small pebble. Describe how the student would find the volume of the pebble using a measuring cylinder, and explain why taking multiple measurements of a short length on a ruler reduces the percentage uncertainty in that measurement.

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

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

Model answer (4 marks)

1. Fill the measuring cylinder with a known volume of water and record the initial reading.
2. Submerge the pebble fully in the water and record the new (final) water level.
3. The volume of the pebble equals the difference between the final and initial water level readings.
4. For a small or short length the absolute uncertainty is a large fraction of the measurement. Taking several measurements and calculating a mean reduces the percentage uncertainty, making the result more reliable and accurate.

Examiner tips

  • Use the word ‘difference’ to show the volume calculation. Show the mean of repeated ruler readings. Explain that the absolute uncertainty stays the same but the relative uncertainty decreases.
  • Use the term ‘percentage uncertainty’ and relate it to the mean of multiple measurements.

Common mistakes

  • Failing to state that the pebble must be fully submerged. Not recognising that the uncertainty is relative to the measurement size. Using the wrong terminology (e.g., ‘average’ instead of ‘mean’).

Mark scheme (4 marks)

  1. Fill the measuring cylinder with a known volume of water and record the initial reading
  2. Submerge the pebble fully in the water and record the new (final) water level
  3. The volume of the pebble equals the difference between the final and initial water level readings
  4. For a small / short length, the uncertainty is a large fraction of the measurement; taking multiple measurements and calculating a mean reduces the percentage uncertainty / makes the result more reliable / accurate

Key terms in this question

volume · measuring cylinder · percentage uncertainty

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