How to Answer IGCSE Physics Calculation Questions

Use this guide to practise How to Answer IGCSE Physics Calculation Questions deliberately. Start with one question, apply the method, mark the evidence and retry the same skill later without notes.

identify → equation → convert → rearrange → substitute → calculate → round → unit → sense-check

Not every question needs all nine steps written at equal length, but each decision should be made deliberately.

Step 1: identify the required quantity

Underline or write:

• what must be calculated
• which values are given
• which values may need to be found first
• the expected unit
• any condition such as constant temperature or ideal transformer

This prevents a student from selecting an equation merely because it contains familiar numbers.

Example:

required: resistance, R
known: potential difference, V; current, I
relationship: R = V / I

Step 2: choose an equation from the current syllabus

Write the relationship before inserting numbers.

Examples:

$v = \frac{s}{t}$

$R = \frac{V}{I}$

$c = \frac{\Delta E}{m\Delta\theta}$

The equation must match both the quantities and the conditions.

For example:

• $pV=\text{constant}$ applies to a fixed mass of gas at constant temperature and is Supplement.
• the efficiency percentage equations are Supplement.
• the transformer voltage-turns ratio is Core.
• a specific latent heat equation is not part of the current 2026 to 2028 syllabus.

Use the current equations list rather than an older formula sheet.

Step 3: convert units before substitution

Write conversions visibly.

1.8 km = 1800 m
4.0 min = 240 s
35 mA = 0.035 A
250 g = 0.250 kg

Volume and area conversions need special care:

• $1\text{ cm}^2 = 10^{-4}\text{ m}^2$
• $1\text{ cm}^3 = 10^{-6}\text{ m}^3$

Do not convert automatically when the equation permits consistent paired units and the requested answer uses them. In $p_1V_1=p_2V_2$, both pressures can remain in kPa and both volumes in litres when the units match between states.

Step 4: rearrange clearly

Rearrange before substituting where practical.

From:

$R=\frac{V}{I}$

make current the subject:

$I=\frac{V}{R}$

From:

$E_k=\frac{1}{2}mv^2$

make speed the subject:

$v=\sqrt{\frac{2E_k}{m}}$

Formula triangles do not handle every relationship reliably. Algebraic rearrangement works for fractions, squares and multi-stage equations.

Step 5: substitute values with units already aligned

Substitution should make the origin of each value clear.

Example:

$I=\frac{12}{24}$

rather than entering the calculation without showing which equation was used.

Use brackets when the denominator contains more than one factor:

$c=\frac{9600}{(0.40)(30)}$

Step 6: calculate accurately

Check calculator mode and entry, especially for:

• standard form
• inverse sine
• squares and square roots
• brackets
• negative exponents

Do not copy a long calculator display as the final answer. Keep sufficient digits internally and round once after the calculation is complete.

Step 7: round appropriately

Use:

• any explicit instruction in the question
• the precision of the supplied data
• sensible scientific reporting

Examples:

• $3.246$ to three significant figures is $3.25$.
• $0.004856$ to two significant figures is $0.0049$.
• $12.00$ communicates greater precision than $12$.

Do not force every answer to two significant figures. Practical tables may also require consistent decimal places based on the instrument and instructions.

Step 8: write the unit

The unit should match the quantity.

Quantity	Common unit
speed	m/s
acceleration	m/s²
force	N
energy	J
power	W
pressure	Pa
current	A
potential difference	V
resistance	Ω
charge	C
frequency	Hz

Compound units matter, for example:

$\text{J/(kg °C)}$

for specific heat capacity.

Step 9: perform a Physics sense-check

Ask:

• Is the sign sensible?
• Is the order of magnitude sensible?
• Is the unit correct?
• Does the answer obey a known limit?
• Does the trend match the situation?

Examples of warning signs:

• efficiency above 100%
• a combined parallel resistance larger than both branch resistances
• a step-down transformer producing a larger secondary voltage
• a student with a weight of only a few newtons
• a gas compressed at constant temperature producing a smaller pressure

A plausible-looking calculator value is not enough.

Worked example 1: Core speed calculation

A cyclist travels 1.8 km in 4.0 minutes. Calculate the average speed in m/s. [3]

Identify

Required: average speed.

Equation

$v=\frac{s}{t}$

Convert

$1.8\text{ km}=1800\text{ m}$

$4.0\text{ min}=240\text{ s}$

Substitute and calculate

$v=\frac{1800}{240}=7.5\text{ m/s}$

Original marking guidance

• converts distance to metres
• converts time to seconds
• calculates $7.5\text{ m/s}$

Worked example 2: Core two-equation chain

A student of mass 52 kg climbs through a vertical height of 4.0 m in 8.0 s. Use $g=9.8\text{ N/kg}$ to calculate the useful power output. [4]

Identify

Power requires energy transferred per unit time. The useful energy gain is gravitational potential energy, which can be obtained through weight and vertical height using Core relationships.

Equations

$W=mg$

$\text{work done}=Fd$

$P=\frac{W}{t}$

The symbol $W$ can mean weight or work depending on context, so keep the labels clear.

Calculate

Weight:

$52\times9.8=509.6\text{ N}$

Useful work:

$509.6\times4.0=2038.4\text{ J}$

Power:

$P=\frac{2038.4}{8.0}=254.8\text{ W}$

A suitably rounded answer is:

$P\approx255\text{ W}$

Sense-check

The answer is a few hundred watts, which is plausible for a person climbing briefly. The unit is watts because the required quantity is power.

Worked example 3: Supplement specific heat capacity

A 0.40 kg metal block receives 9.6 kJ and its temperature rises by 30 °C. Calculate its specific heat capacity. [4]

Equation

$c=\frac{\Delta E}{m\Delta\theta}$

This equation is Supplement.

Convert

$9.6\text{ kJ}=9600\text{ J}$

Substitute

$c=\frac{9600}{(0.40)(30)}$

Calculate

$c=800\text{ J/(kg °C)}$

Original marking guidance

• converts 9.6 kJ to 9600 J
• selects the specific heat capacity equation
• substitutes mass and temperature change correctly
• obtains $800\text{ J/(kg °C)}$

Worked example 4: Core transformer ratio

A transformer has 1200 turns on the primary coil and 80 turns on the secondary coil. The primary voltage is 240 V. Calculate the secondary voltage. [3]

Equation

$\frac{V_p}{V_s}=\frac{N_p}{N_s}$

The voltage-turns relationship is Core.

Rearrange

$V_s=\frac{V_pN_s}{N_p}$

Substitute

$V_s=\frac{(240)(80)}{1200}=16\text{ V}$

Sense-check

The secondary has fewer turns, so it should have a lower voltage. The result is consistent with a step-down transformer.

Worked example 5: Supplement gas law with paired units

A fixed mass of gas occupies 80 cm³ at 120 kPa. Its temperature remains constant while the pressure increases to 200 kPa. Calculate the new volume. [3]

Equation

$p_1V_1=p_2V_2$

Rearrange

$V_2=\frac{p_1V_1}{p_2}$

Substitute

$V_2=\frac{(120)(80)}{200}=48\text{ cm}^3$

No conversion to pascals or cubic metres was required because the pressure units match and the volume remains in cm³.

Sense-check

Pressure increased at constant temperature, so volume should decrease. The result is plausible.

Common calculation error patterns

Correct equation, wrong tier

A student may memorise a relationship that is not required for the route or is removed from the current syllabus.

Fix: label equations Core or Supplement and use the current source of truth.

Correct values, mixed units

Fix: create a conversion block before substitution.

Correct equation, inverted rearrangement

Fix: rearrange symbolically and check the expected trend.

Correct method, missing unit

Fix: identify the expected unit in Step 1 and write it with the final number.

Correct calculator entry, unjustified precision

Fix: keep working digits and round once using the data and instructions.

Implausible answer accepted

Fix: use known limits and trends before moving on.

A practice scoring grid

For each original practice question, award process credit only when the step is visible:

Check	Completed?
required quantity identified
current equation selected
units aligned
rearrangement correct
substitution visible
arithmetic correct
rounding justified
unit present
answer physically plausible

This grid is a training tool, not an official Cambridge mark scheme. The actual allocation varies by question.

How should the method become automatic?

Use short mixed sets rather than repeating only one equation.

1. complete five calculations from different topics
2. classify each error
3. practise the weak process step
4. answer another mixed set
5. retry the failed questions after a delay
6. test the method inside a timed component

The Maths for IGCSE Physics guide isolates unit, algebra, ratio, graph and standard-form skills. The original question bank provides fresh calculations across all six topics.

Can tutoring help?

A tutor can help distinguish:

• concept misunderstanding
• wrong equation choice
• algebra
• units
• calculator use
• checking

The lesson should then require the student to reproduce the corrected process independently. No single session guarantees that the habit is installed permanently.

A dependable calculation answer is not created by memorising a five-word slogan. It comes from making each Physics and Mathematics decision visible, checking it and applying it again in a new context.