A student measures the mass of a small stone as 0.0035 kg. Explain how the student could express this measurement using a more appropriate prefix, and state which prefix and unit combination should be used.

Edexcel GCSE Physics (1PH0) — 1.1 Key concepts of physics · Explain · 4 marks · View as Markdown

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

A student is recording measurements in a physics experiment. They write down the mass of a small stone as 0.0035 kg.

Model answer (4 marks)

The value 0.0035 kg has three leading zeros after the decimal point, making it inconvenient to read and use. A prefix can replace the power of ten to give a more manageable number. The appropriate prefix is milli (m), which is ×10⁻³. Thus 0.0035 kg = 3.5 g (or 3.5 mg if the unit is kept in kilograms).

Examiner tips

  • Use the word ‘prefix’ and give the symbol (m) and its factor (×10⁻³).
  • Show the conversion step: 0.0035 kg ×10³ g/kg = 3.5 g.
  • Mention that 3.5 mg is also acceptable if the unit remains kg.
  • Keep the answer concise and use UK spelling.

Common mistakes

  • Writing 0.0035 kg as 3.5 kg (wrong factor).
  • Using the wrong prefix such as centi (c) or kilo (k).
  • Failing to state the unit after the new number.

Mark scheme (4 marks)

  1. Identifies that the measurement 0.0035 kg contains an inconvenient number of decimal places / zeros, making it difficult to work with or read clearly
  2. States that a prefix can be used to replace the power of ten / scale the unit to give a more manageable number
  3. Correctly identifies the prefix 'milli' (m) as appropriate, equivalent to ×10⁻³
  4. States the correctly expressed value with the correct unit: 3.5 g or 3.5 milligrams (mg) — accept grams as the unit here since 1 g = 10⁻³ kg; award mark for 3.5 mg with unit stated

Key terms in this question

prefix

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