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Physics is a quantitative subject, and calculations typically account for around 40% of the marks across all three papers. The difference between a good grade and a great grade often comes down to calculation technique — not whether you can solve the problem, but whether you present your solution in a way that earns every available mark.
Every calculation in physics should follow this structure:
This structure is essential because examiners award method marks as well as answer marks. If you make an arithmetic error but your method is clearly correct, you will still earn method marks. If you only write the final answer and it is wrong, you score zero.
graph TD
A["Read the question"] --> B["Identify known quantities"]
B --> C["Choose the correct equation"]
C --> D["Write equation in symbols"]
D --> E["Substitute values WITH units"]
E --> F{"Need to rearrange?"}
F -->|Yes| G["Rearrange algebraically"]
F -->|No| H["Calculate"]
G --> H
H --> I["Write answer"]
I --> J["Add correct units"]
J --> K["Round to appropriate sig figs"]
K --> L{"Does answer seem\nreasonable?"}
L -->|Yes| M["Move to next question"]
L -->|No| N["Re-check working"]
N --> B
Many students lose marks because they cannot rearrange equations confidently. Practise rearranging the key equations from your equation sheet:
Example: Find the velocity of a wave given that the wavelength is 0.50 m and the frequency is 440 Hz.
Example: Find the resistance of a component if the current is 2.5 A and the potential difference is 12 V.
Example (multi-step): A satellite orbits Earth at radius r. Show that orbital speed v = √(GM/r).
This kind of multi-step rearrangement is common in Paper 2 and Paper 3. Practise combining two equations into one by eliminating a common variable.
Physics units are not optional decoration — they are an integral part of the answer. Including units at every stage of your working helps you:
| From | To | Multiply by | Common context |
|---|---|---|---|
| cm | m | × 0.01 | Plate separation, wire diameter |
| mm | m | × 0.001 | Slit width, wire extension |
| km | m | × 1000 | Orbit radius, distance |
| g | kg | × 0.001 | Small masses in experiments |
| °C | K | + 273 | Gas law calculations |
| kJ | J | × 1000 | Energy transfers |
| mA | A | × 0.001 | Small currents |
| μF | F | × 10⁻⁶ | Capacitance |
| nF | F | × 10⁻⁹ | Small capacitance |
| nm | m | × 10⁻⁹ | Wavelengths of light |
| eV | J | × 1.60 × 10⁻¹⁹ | Particle energies |
| hours | s | × 3600 | Time in orbit/decay problems |
Always convert to SI base units before substituting into equations. A useful habit is to write the conversion explicitly: "d = 5.0 cm = 5.0 × 10⁻² m" — this earns credit and reduces the chance of error.
The general rule for Edexcel Physics is to give your answer to the same number of significant figures as the data in the question, or to 2–3 significant figures if not specified. Never give an answer to more significant figures than the least precise piece of data.
Important exceptions:
| Data precision | Your answer should be | Example |
|---|---|---|
| 2 s.f. | 2 s.f. | Data: 4.5 m → Answer: 12 J |
| 3 s.f. | 3 s.f. | Data: 4.50 m → Answer: 12.3 J |
| "Show that X = 4.5" | At least 3 s.f. | Your answer: 4.52 |
| Mixed (2 and 3 s.f.) | 2 s.f. (least precise) | Use the lower value |
| Not specified | 2–3 s.f. | Default to 3 s.f. |
Many physics quantities are very large or very small. Standard form (scientific notation) keeps your answers manageable:
When multiplying or dividing in standard form, deal with the coefficients and powers of 10 separately:
(3.0 × 10⁸) × (2.0 × 10⁻³) = 6.0 × 10⁵
For uniform acceleration problems, identify the three known quantities and the one unknown, then select the appropriate equation:
Always define your positive direction and be careful with signs — acceleration due to gravity is positive downward if you define downward as positive, and negative if you define upward as positive.
Examiners distinguish between:
If a question is worth 3 marks, it might be structured as M1 A1 A1 (one method mark and two answer marks) or M1 M1 A1 (two method marks and one answer mark). This is why showing working is so important — in a 3-mark question, you can still score 2 marks even with the wrong final answer.
Question (3 marks): Calculate the kinetic energy of a car of mass 1200 kg travelling at 25 m s⁻¹.
Student's answer:
Score: 1 out of 3. But if the student had written:
The lesson: writing "25²" explicitly rather than jumping to a number protects you from losing marks.
After completing a calculation, ask yourself: "Does this answer make sense?" If you calculate the speed of a car as 5 × 10⁶ m s⁻¹, something has gone wrong — that is faster than the speed of light. If you calculate the mass of a ball as 0.00002 kg, check whether you forgot to convert grams to kilograms or millimetres to metres.
| Quantity | Typical value |
|---|---|
| Walking speed | ~1.5 m s⁻¹ |
| Car on a motorway | ~30 m s⁻¹ |
| Speed of sound in air | ~340 m s⁻¹ |
| Mains voltage (UK) | 230 V |
| g at Earth's surface | 9.81 m s⁻² |
| Atmospheric pressure | ~1 × 10⁵ Pa |
| Room temperature | ~293 K (~20°C) |
| Mass of an electron | 9.11 × 10⁻³¹ kg |
| Charge on an electron | 1.60 × 10⁻¹⁹ C |
| Visible light wavelength | 400–700 nm |
If your answer is wildly outside these ranges for a typical problem, re-check your working before moving on.
Calculation is the load-bearing skill of A-Level physics. Across the three Edexcel 9PH0 papers a candidate will perform somewhere between 35 and 50 distinct numerical operations under time pressure, ranging from a one-line substitution into a stated formula through to a five-step derivation that combines two principles and ends in an exact algebraic result. The strategy that wins these marks is not "be quick with a calculator" — it is a disciplined sequence of equation, knowns, substitution, calculation, units, sig figs applied without exception, even on the easiest one-mark items. The sections below break that sequence into a usable game plan.
Calculation questions on Edexcel 9PH0 fall into four recognisable types, and the route through each is different.
| Calculation type | Typical mark band | Character | Where it appears |
|---|---|---|---|
| Single-step | 1-2 marks | One equation, one substitution, one unit | Front of every paper, scattered through the middle |
| Multi-step | 3-5 marks | Two or three equations chained, often with a rearrangement before substitution | Middle band of all three papers |
| Derivation | 4-6 marks | Algebraic manipulation from a stated principle to a target expression | Most reliably on Paper 2 and synoptic items on Paper 3 |
| "Show that" | 2-4 marks | Stated numerical answer; the marks are entirely for the route | All three papers, increasingly common in middle band |
Across recent Edexcel 9PH0 papers, calculation-based questions consistently account for between 55 and 70 per cent of the total marks. Paper 1 sits at the upper end of that range because mechanics, circuits and waves are calculation-dense; Paper 2 sits in the middle because field, oscillation and thermodynamic questions blend numerical work with explanation; Paper 3 sits at the lower end because the practical-skills block trades calculation marks for graph-drawing, uncertainty and evaluation marks. The strategic implication is direct: a candidate who is fluent and tidy in calculation has already secured the majority of the marks on Papers 1 and 2 before a single explanation is written.
The four types nest. A multi-step calculation typically opens with a single-step substitution to find an intermediate value, then chains a second equation onto that value. A derivation often ends in a "show that" target, where the final numerical answer is stated and the entire mark-load lives in the algebraic route. Recognising which type you are in inside the first 20 seconds of reading the question is what determines whether you set out the work as numerical substitution or as algebraic manipulation — and getting this wrong wastes 90 seconds of writing in the wrong register.
The other defining feature of Edexcel calculation questions is that the data and formulae booklet is always available. Every constant you might need (g, c, e, k, h, ε₀, μ₀, R, k_B, N_A) and every standard equation that is not part of the assumed set (capacitor energy, lens formula, gravitational potential, simple harmonic motion solutions) is listed there. The candidate who has not opened the booklet in the first ten minutes of the paper has already burned a strategic advantage: knowing where on the booklet a relationship lives turns a 40-second hunt into a 5-second lookup.
Calculation-heavy questions follow the same 1.2 min per mark anchor as the rest of the paper, but the internal distribution of that time is not uniform — most of it is spent reading and rearranging, not computing.
| Mark value | Target time | Realistic upper bound | Time split |
|---|---|---|---|
| 1 mark | 1 min | 1.5 min | 30s read, 20s substitute, 10s write |
| 2 marks | 2.5 min | 3 min | 45s read, 60s substitute and calculate, 45s write with unit |
| 3 marks | 3.5 min | 4.5 min | 60s read and rearrange, 90s calculate, 60s units and sig figs |
| 4 marks | 5 min | 6 min | 90s plan and rearrange, 2 min calculate, 90s check |
| 5 marks | 6 min | 7.5 min | 2 min plan, 2.5 min calculate, 90s check and present |
| 6 marks | 7 min | 9 min | 2.5 min plan, 3 min calculate, 90s check |
| 8 marks | 10 min | 12 min | 3 min plan, 5 min execute, 2 min check |
The figure to anchor against is 1.2 minutes per mark for calculation-heavy questions. A 4-mark calculation should consume roughly 5 minutes start-to-finish, including reading the stem, identifying the equation, performing the rearrangement, substituting values, computing, and writing the answer with the correct unit and significant figures. A 6-mark calculation deserves up to 7 minutes — but if you are still rearranging at the 9-minute mark you have stolen time from somewhere else.
A consequence of the 1.2 min per mark figure that is worth internalising: roughly half of a calculation question's time should be spent before the calculator comes out. Candidates who reach for the calculator within the first 30 seconds of a 4-mark calculation almost always end up substituting in the wrong form, mishandling a unit conversion, or solving for the wrong variable. Slowing down to write the equation, identify what is given and what is sought, and rearrange algebraically before any number is typed protects against the most common slips.
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