A physics technician records the following measurements during an experiment: a length of 0.0035 m, a mass of 0.000 002 kg, and a time of 15 000 000 ns. Describe how each measurement can be expressed using an appropriate SI prefix, and explain why using SI prefixes is important in scientific communication.

AQA A-Level Physics (7408) — 3.1.1 Use of SI Units and Their Prefixes · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

0.0035 m = 3.5 mm (millimetre) or 3500 µm (micrometre) – both use the correct SI prefix.
0.000 002 kg = 2 µg (microgram) or 0.002 mg (milligram) – the prefix matches the magnitude.
15 000 000 ns = 15 ms (millisecond) or 0.015 s – the prefix gives a convenient unit.
Using SI prefixes removes long strings of zeros, making numbers easier to read and write.
SI prefixes provide a common, unambiguous system so scientists worldwide can communicate measurements consistently.

Examiner tips

  • Show each conversion with the correct prefix and unit. Explain the benefit of fewer zeros and global consistency. Use UK spelling (millimetre, millisecond).

Common mistakes

  • Using the wrong prefix (e.g. 3.5 µm instead of 3.5 mm). Forgetting to include the unit after the number. Mixing metric and imperial units (e.g. writing 15 ms as 15 s).

Mark scheme (5 marks)

  1. 0.0035 m expressed correctly using a prefix (e.g. 3.5 mm or 3500 µm)
  2. 0.000 002 kg expressed correctly using a prefix (e.g. 2 µg or 0.002 mg or 2000 ng)
  3. 15 000 000 ns expressed correctly using a prefix (e.g. 15 ms or 0.015 s)
  4. SI prefixes reduce the number of zeros / make values easier to read or write
  5. SI prefixes allow scientists worldwide to communicate measurements consistently / without ambiguity

Key terms in this question

SI prefix

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