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The three-line layout that secures the marks
Calculation marks are earned by method, not only by the final number, so lay every calculation out the same way.
- Write the formula in symbols first.
- Substitute the values with their units.
- State the answer with its unit and to a sensible number of significant figures.
Worked example, finding force:
- Formula: F = ma
- Substitution: F = 3 kg × 4 m s⁻²
- Answer: F = 12 N (2 s.f.)
Every line that carries numbers also carries units. Laying it out this way means that even if the final figure slips, the method marks are still there for the formula and the correct substitution.
Units and prefixes: convert before you compute
Most lost calculation marks trace back to units, not arithmetic.
- Convert quantities to consistent SI units before substituting: 20 cm = 0.20 m, 500 g = 0.5 kg, 5 kV = 5000 V, 250 ms = 0.25 s.
- Carry the units through the working, not just at the end.
- Check the final unit makes physical sense: a speed should come out in m s⁻¹, an energy in J.
Worked example, finding density:
- Formula: ρ = m ÷ V
- Substitution: ρ = 0.5 kg ÷ 0.002 m³
- Answer: ρ = 250 kg m⁻³
If the unit at the end looks wrong, the substitution usually needs rechecking.
Significant figures, and no formula sheet
SPM Physics 4531 has no formula sheet, so every formula must be recalled from memory. Build that recall as you practise, and never assume a formula will be given in the exam.
- Keep full precision during the working and round only the final answer.
- Give the answer to a sensible number of significant figures, usually matching the data, often 2 or 3.
- Always write the unit; a numerically correct answer with no unit typically loses the final mark.
Worked example, finding voltage:
- Formula: V = IR
- Substitution: V = 2 A × 5 Ω
- Answer: V = 10 V
A scientific calculator is allowed, so use it for the arithmetic while you supply the formula and the units yourself.