IGCSE · 25 September 2026 · 6 min read
Cambridge IGCSE Physics Paper 4 calculations: show a complete method
A correct equation is only the beginning of a useful calculation. You also need to decide which quantities belong in it, whether the units are compatible and whether the result answers the question. A transparent method makes errors easier to find and gives the marker evidence of your reasoning.
This article targets Cambridge IGCSE Physics 0625 Extended. In the 2026–2028 syllabus, Paper 4 is a 75-minute, 80-mark theory paper covering Core and Supplement content. It contains structured and short-answer questions, including explanations; it is not a calculation-only examination. The examples here are original practice material.
Translate the situation into quantities
Begin with a short data line: mass = 0.40 kg, initial speed = 6.0 m/s, final speed = 0. Name an unknown with a symbol before looking for an equation. This is particularly helpful when a question supplies more information than one step needs. A word such as constant, stationary, insulated or negligible may determine which physical model is appropriate.
Avoid selecting a formula because it contains the largest number of given values. Ask what process connects the starting and ending situations: an energy transfer, a resultant force, a pressure difference or an electrical relationship. The same object can appear in several different models, but the question decides which quantity matters.
Worked example: use energy to find a stopping force
A 0.40 kg toy car moves at 6.0 m/s before a constant resisting force brings it to rest over 1.2 m on a horizontal track. Its initial kinetic energy is Eₖ = ½mv² = ½ × 0.40 × 6.0² = 7.2 J. The kinetic energy decreases to zero, so the work done against the resisting force is 7.2 J. Using W = Fd gives F = 7.2 / 1.2 = 6.0 N, opposite the motion.
The reasoning includes an assumption: the stated resisting force accounts for the energy transferred from the car's kinetic store. We have not used 7.2 J as a force; the stopping distance connects work to force. As a check, F = ma gives an acceleration of magnitude 15 m/s². With these numbers, the car stops in 0.40 s and its average speed of 3.0 m/s produces the stated 1.2 m distance.
Worked example: convert volume before calculating density
A sample has mass 540 g and volume 200 cm³. In the units given, density = 540 / 200 = 2.7 g/cm³. If the question requires kg/m³, convert both quantities: 540 g = 0.540 kg and 200 cm³ = 200 × 10⁻⁶ m³ = 2.00 × 10⁻⁴ m³. The density is therefore 0.540 / (2.00 × 10⁻⁴) = 2700 kg/m³.
The common conversion error is treating a volume as a length. One centimetre is 10⁻² m, so one cubic centimetre is (10⁻²)³ = 10⁻⁶ m³. Writing the conversion explicitly prevents a factor-of-ten or factor-of-a-thousand answer from passing unnoticed. Both density answers describe the same material; they simply use different unit systems.
Show rearrangement and keep useful precision
If you need time from P = E / t, rearrange to t = E / P before substituting. If you need speed from Eₖ = ½mv², use v = √(2Eₖ / m); remembering the square root matters as much as remembering the original equation. One clear algebraic step can prevent a series of confused calculator operations.
Carry extra digits through an intermediate result and round the final result as the question requires. Do not replace a calculated 0.267 A with 0.3 A halfway through a longer calculation unless that precision is appropriate. Equally, a long calculator display is not a claim of measurement precision: write a sensible final value and the correct unit.
Treat checks as part of the solution
Check the dimensions: energy divided by time produces power, while force divided by area produces pressure. Check the trend: increasing the contact area at fixed force should decrease the pressure. Check the sign or direction when relevant: a braking acceleration is opposite the motion. These checks test the physical meaning rather than repeating the same arithmetic.
If a later part uses an earlier result, label that result clearly and continue with a consistent method. Marking arrangements depend on the individual question, so showing working does not guarantee a particular number of method marks. It does provide evidence of correct steps and makes it easier for you to diagnose a mistaken substitution afterwards.
Independent practice: pressure with an area conversion
A block exerts a downward force of 60 N over a contact area of 30 cm². Calculate its pressure in pascals, then predict the pressure when the same force acts over 15 cm². First convert 30 cm² to 30 × 10⁻⁴ = 0.0030 m². Then p = F / A = 60 / 0.0030 = 20 000 Pa. Halving the area doubles the pressure to 40 000 Pa.
If you obtained 2 Pa, you divided by the numerical area in cm² while labelling the result N/m². If you predicted a smaller pressure for the smaller area, revisit the meaning of force per unit area. Record both the conversion and the reasoning in your correction. Reattempt with a fresh force and area in a later revision session: your corrected method should work without the original solution beside you.
Questions, explained
Choose a question for a direct answer, then explore the explanation and supporting resources. Each answer has its own link to save or share.
Do I need to show working in Cambridge IGCSE Physics Paper 4?
Show the relationship, substitution and any important conversions or rearrangement. This makes your method visible and helps you check it. Some questions can award credit for valid intermediate steps, but the allocation depends on that question's mark scheme. Avoid assuming that an unsupported final answer or a memorised formula always receives the same credit.
Which unit conversions cause problems in physics calculations?
Area and volume conversions deserve particular care because the length conversion is squared or cubed. For example, 1 cm² = 10⁻⁴ m² and 1 cm³ = 10⁻⁶ m³. Also check minutes to seconds, grams to kilograms and prefixes such as milli and kilo. Write the converted quantity beside its original value before substituting into an SI equation.
Can I prepare for Paper 4 using calculations only?
No. Cambridge 0625 Paper 4 includes short-answer and structured questions across the Extended content, so preparation also needs explanations, definitions, diagrams and interpretation. After solving a calculation, explain the trend in words: why does pressure fall when area rises, or why does doubling speed quadruple kinetic energy? This connects the numerical method to the physical understanding.