A student writes down four statements about physical quantities and their units. For each statement, explain whether the student is correct or incorrect, giving a reason. Statement 1: 'Momentum is measured in kg m/s.' Statement 2: 'The spring constant has the unit metres per newton (m/N).' Statement 3: 'Gravitational field strength is measured in N/kg.' Statement 4: 'The moment of a force is measured in newton metres (Nm).'
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Statement 1: Correct – momentum = mass × velocity, so the unit is kg m s⁻¹.
Statement 2: Incorrect – the spring constant k = F/x, so its unit is N m⁻¹ (newtons per metre), not m N⁻¹.
Statement 3: Correct – gravitational field strength is force per unit mass, giving units of N kg⁻¹.
Statement 4: Correct – the moment (torque) = force × perpendicular distance, so the unit is N m (newton metres).
Statement 2: Incorrect – the spring constant k = F/x, so its unit is N m⁻¹ (newtons per metre), not m N⁻¹.
Statement 3: Correct – gravitational field strength is force per unit mass, giving units of N kg⁻¹.
Statement 4: Correct – the moment (torque) = force × perpendicular distance, so the unit is N m (newton metres).
Examiner tips
- Use the definition of each quantity to state the correct unit; show the formula if needed.
- Check that the unit matches the SI base units (kg, m, s).
- Avoid mixing up reciprocal units – e.g. N/m vs m/N.
Common mistakes
- Confusing the spring constant unit as m/N instead of N/m.
- Writing the unit for momentum as kg m² s⁻¹ (incorrect).
- For torque, writing Nm² or omitting the unit entirely.
Mark scheme (4 marks)
- Statement 1 is correct: momentum = mass × velocity, so the unit is kg m/s.
- Statement 2 is incorrect: the spring constant k = F/x, so its unit is N/m (newtons per metre), not m/N.
- Statement 3 is correct: gravitational field strength is the force per unit mass, giving units of N/kg.
- Statement 4 is correct: moment = force × perpendicular distance from pivot, so the unit is N × m = Nm.
Key terms in this question
momentum · spring constant · gravitational field strength · moment
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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