How to Answer IGCSE Physics Calculation Questions

Calculations carry roughly 40% of the marks on IGCSE Physics theory papers, and they are the most predictable marks on the paper. Every calculation question yields to the same five-step routine: equation, substitute, rearrange, unit, significant figures. This page teaches the routine, then proves it on three worked 0625-style examples.

What is the method for IGCSE Physics calculations?

Write the standard equation, substitute the numbers with units converted, rearrange for the unknown, state the answer with its unit, and round to 2 or 3 significant figures. Five lines, every time, no shortcuts. The routine exists because mark schemes pay for visible steps, not just final answers.

Here it is as the checklist we drill:

1. Equation. Write the standard form in symbols, e.g. $P = IV$. This alone often earns M1.
2. Substitute. Put the numbers in, converting units first: cm → m, g → kg, minutes → seconds, kW → W.
3. Rearrange. Isolate the unknown. With numbers already in place, this is one arithmetic move.
4. Unit. Write the SI unit on the answer line. No unit usually means no A1.
5. Significant figures. Round once, at the end, to 2 or 3 sig figs.

Two non-negotiables sit underneath. First, never round mid-calculation; carry the full calculator value through. Second, show working even when the calculation feels trivial, because method marks survive arithmetic slips and final answers do not.

Worked example 1: a single-step Core calculation

A student of mass 52 kg climbs stairs of vertical height 4.0 m in 8.0 s. Calculate the student’s useful power output. ($g = 9.8\ \text{N/kg}$) [3]

Equation: work done $W = Fd$, where the force is the weight, and $P = \dfrac{W}{t}$.

Substitute and solve in stages:

• Weight: $W = mg = 52 \times 9.8 = 509.6\ \text{N}$
• Work done: $W = Fd = 509.6 \times 4.0 = 2038.4\ \text{J}$
• Power: $P = \dfrac{2038.4}{8.0} = 254.8\ \text{W}$

Answer with unit and sig figs: $P = 250\ \text{W}$ (2 s.f.)

Mark scheme logic: M1 for $mg$ or $509.6\ \text{N}$ seen, M1 for work or energy ÷ time method, A1 for 250-255 W with unit. A student who blanked on the final division still banks two marks for the visible method.

Worked example 2: rearrangement with a unit trap

A heater supplies 33 600 J to 0.80 kg of water and the temperature rises from 28 °C to 38 °C. Calculate the specific heat capacity of water. [3]

Equation: $c = \dfrac{\Delta E}{m\Delta\theta}$

Substitute: the temperature change is $38 - 28 = 10\ ^\circ\text{C}$, not 38. This is the trap; $\Delta$ means change.

$c = \dfrac{33\,600}{0.80 \times 10}$

Solve: $c = \dfrac{33\,600}{8.0} = 4200$

Answer: $c = 4200\ \text{J/(kg}\ {}^\circ\text{C)}$

The unit here is itself worth attention: joules per kilogram per degree Celsius. Mark schemes for thermal questions regularly reserve the final mark for that compound unit. M1 substitution with $\Delta\theta = 10$, A1 for 4200, B1 or built-in credit for the unit depending on the paper.

Worked example 3: a two-equation chain (Core equations, Extended-style demand)

A 1.2 kW kettle runs from the 240 V mains. Calculate the resistance of its heating element. [4]

No single equation links P, V and R on the Core list, so chain two.

Equations: $P = IV$, then $R = \dfrac{V}{I}$.

Step 1, find the current. Convert first: $1.2\ \text{kW} = 1200\ \text{W}$.

$I = \dfrac{P}{V} = \dfrac{1200}{240} = 5.0\ \text{A}$

Step 2, find the resistance.

$R = \dfrac{V}{I} = \dfrac{240}{5.0}$

Answer: $R = 48\ \Omega$ (2 s.f.)

Marking: M1 for $P = IV$ used correctly, A1 for $5.0\ \text{A}$, M1 for $R = \dfrac{V}{I}$, A1 for $48\ \Omega$ with unit. Notice the kW conversion happened before any substitution. Students who substitute 1.2 instead of 1200 produce $0.005\ \text{A}$ and an absurd $48\,000\ \Omega$, and a quick sanity check, “does a kettle really have $48\ \text{k}\Omega$ of resistance?”, catches it.

Why do students lose calculation marks when they know the physics?

Almost always for one of five mechanical reasons: missed unit conversions, rounding too early, missing units on the answer, formula-triangle dependence failing on equations like $E_k = \dfrac{1}{2}mv^2$, and skipping written working. None of these is a physics problem. All five disappear with the routine above applied every single time.

The conversion list worth tattooing onto your pencil case:

The cm³ row catches the most students, usually in density questions. When the data uses g and cm³, you may keep both and answer in g/cm³; mixing systems is what kills the mark.

“My child understands the topic but the exam marks don’t show it.” We hear this in almost every first conversation with parents, and the diagnosis is usually mechanical, not conceptual: steps done in the head, units dropped, answers rounded at step two. In our 1-to-1 online classes, tutors mark calculation working line by line the way a Cambridge examiner does, so students see exactly which line paid and which line cost. One 1.5-hour session is usually enough to install the five-step habit; the free 1-hour trial lesson can start that process with your child’s own past paper.

Building the habit before exam day

Train the routine on volume, not difficulty. Take any past Paper 3 or 4, do only the calculation parts, and grade yourself one point per step: equation written, units converted, working shown, unit on answer, sensible sig figs. Twenty questions scored this way teaches more than five full papers done loosely.

In the final week, add the sanity check as a sixth step: is the answer physically plausible? A car at 4000 m/s, a student weighing 5 N, a kettle drawing 0.005 A; each absurd value is a free invitation to find the slipped decimal.

Memorise the equations, run the five steps, round once at the end. Calculation questions stop being a risk and become the most reliable 40% of your theory paper.