A-Level Physics Calculation Rules: Which Formulas to Memorize, Unit Conversions, Significant Figures, and How to Answer "Show That" Questions

For calculation questions in A-Level Physics, students from mainland China are usually not afraid of the "calculation" part, but they often lose points on seemingly minor details: failing to convert units, using the wrong number of significant figures, memorizing formulas incorrectly, skipping steps in "Show that" questions, and forgetting to write units after calculations. These are all explicitly regulated. Based on the Cambridge 9702 (2025–2027) and Edexcel IAL Physics (3rd edition) syllabuses, this article clarifies the rules and answering methods for calculation questions. For the exam structure, see How A-Level Physics is Tested; for A-Level difficulties and practicals, see articles 177 and 185.

1. Formulas: Which Are Provided and Which to Memorize

• Cambridge 9702: The syllabus states that data and formulas will be printed on page 2 of Papers 1 and 2, and pages 2 and 3 of Paper 4. However, the formula sheet does not include all formulas: many syllabus learning objectives state "recall and use," such as the photoelectric equation hf = Φ + ½mv²_max, Hubble's law v ≈ H₀d, the attenuation formula for ultrasound and X-rays I = I₀e^−μx, and Wien's displacement law λ_max ∝ 1/T. None of these are on the formula sheet. Any formula marked with "recall" must be memorized;
• Edexcel IAL: The syllabus specifies that students do not need to memorize formulas. The formulas listed in the syllabus appendix will be provided in every exam, and any other required formulas will be given in the questions; constants will also be provided on every paper.

Recommendation (based on experience): Cambridge candidates should print out the formula sheet and mark each item in the syllabus as either "on the formula sheet" or "to be memorized." Although Edexcel candidates do not need to memorize them, they should familiarize themselves with the location and symbols of the formula sheet to find them quickly during the exam.

2. Constants: What Value to Use for g

Both exam boards provide the acceleration due to gravity as g = 9.81 m s⁻² on their Physics papers, along with other constants such as the speed of light, elementary charge, and Planck constant. Note: The Cambridge Mathematics 9709 Mechanics paper specifies g = 10 m s⁻² (see How to Study A-Level Mechanics). Students taking both Math and Physics should avoid mixing up the values between the two subjects.

3. Units: Mandatory Syllabus Requirements

Chapter 1 of Cambridge 9702, "Physical Quantities and Units," requires:

• All physical quantities consist of a numerical value and a unit;
• Memorize the SI base quantities and their units: mass (kg), length (m), time (s), electric current (A), and temperature (K);
• Express derived units as products or quotients of SI base units;
• Use SI base units to check the homogeneity of physical equations;
• Memorize and use prefixes: pico (p), nano (n), micro (μ), milli (m), centi (c), deci (d), kilo (k), mega (M), giga (G), and tera (T);
• Make reasonable estimates for physical quantities covered in the syllabus.

The syllabus's mathematical requirements also state that units must be consistent when substituting into physical equations, and the dimensional homogeneity of equations must be checked.

An example of a homogeneity check (created by us to demonstrate the method): Check if the units of the wave speed formula on a string v = √(T/μ) are consistent, where T is tension (N) and μ is linear mass density (kg m⁻¹).

• N = kg m s⁻², so the unit of T/μ is (kg m s⁻²) ÷ (kg m⁻¹) = m² s⁻²;
• Taking the square root gives m s⁻¹, which is exactly the unit of speed, so the formula is homogeneous in terms of units.

Common question formats for this type of problem are "express the unit of a quantity in SI base units" or "show that an equation is homogeneous." The key is to break down every derived unit (N, J, W, C, V, Ω, etc.) into kg, m, s, A, and K.

4. Significant Figures: Syllabus Regulations

Cambridge 9702 syllabus regulations: For quantities calculated from measured values (excluding addition and subtraction), the number of significant figures usually depends on the measured value with the fewest significant figures; if it has n figures, the result should be written to n or n + 1 figures. Syllabus example: if voltage is measured to 2 figures and current to 4 figures, the calculated resistance should be written to 2 or 3 figures, not 1 or 4 figures. The syllabus's mathematical requirements also state that significant figures should neither be unnecessarily lost nor retained beyond a reasonable range during calculations.

Recommendation (based on experience): Keep a few extra significant figures in intermediate steps (or store them in the calculator), and round according to the rules only at the final step; when the data given in the question are all to 3 significant figures, the answer should generally be given to 2 or 3 figures.

5. Command Words: Calculate, Determine, Show that

Definitions in the Cambridge 9702 syllabus:

• Calculate: Work out a result from given facts, figures, or information;
• Determine: Find the answer using available information (often requiring data to be taken from graphs or previous results);
• Show (that): Provide structured evidence to derive the given result.

Be especially careful with "Show that" questions (recommendation based on experience): The answer is already given, and examiners grade the process. You must write out the formula, the substituted values (with units), and the calculation steps. The final calculated result should be given to one more significant figure than the value provided in the question to prove that you calculated it rather than just copying it. Subsequent parts of the question will usually use this result, so you can continue calculating using the value given in the question.

6. Mathematical Requirements

The Cambridge 9702 syllabus lists the math skills required for both AS and A Level: scientific notation; using a calculator for powers, roots, trigonometric and inverse trigonometric functions; checking the order of magnitude of results using approximations; rearranging formulas to change the subject; solving quadratic equations in one variable and systems of linear equations in two variables; area, volume, Pythagoras' theorem, the sine rule, and the cosine rule; vector addition and resolution; and gradients, intercepts, tangents, and areas of graphs. A Level also requires: converting between radians and degrees, small-angle approximations (sin θ ≈ tan θ ≈ θ, cos θ ≈ 1), and indices and logarithms (lg and ln).

The Edexcel IAL Physics syllabus specifies that at least 40% of the marks across the entire qualification will assess mathematical skills at Level 2 or above, for example, at least 32 marks per paper in Units 1 and 2, and at least 36 marks per paper in Units 4 and 5.

7. How to Write a Calculation Question Answer

Here are recommendations based on experience:

1. Write the formula first: Write the formula used in symbols, e.g., P = IV;
2. Convert units before substituting: Convert mA, kJ, cm, etc., to SI units first, and write the numerical values when substituting;
3. Show the calculation process: For multi-step calculations, write down the intermediate result for each step;
4. Include units in the answer and use appropriate significant figures;
5. Check the order of magnitude: The syllabus requires using approximations to check results. For example, if you calculate a person's kinetic energy to be 10⁶ J, you should go back and check;
6. Include direction for vectors: For velocity, force, field strength, etc., write the direction when required by the question.

8. Common Reasons for Losing Marks

Here is a summary based on experience:

• Failing to convert units: Substituting cm, g, mA directly;
• Forgetting to write units in the final answer;
• Too many significant figures: Copying the long string of numbers directly from the calculator;
• Only writing the result for "Show that" questions;
• Cambridge candidates not memorizing "recall" formulas;
• Wrong angle mode: Calculator set incorrectly between radians and degrees;
• Mixing up lg and ln in logarithm questions.

9. How to Practice

1. Organize a formula list: Cambridge candidates should categorize them as "on the formula sheet / to be memorized";
2. Practice units specifically: Pick a few derived units from each chapter and break them down into SI base units;
3. Practice estimation: The syllabus requires estimating common physical quantities, such as a person's mass, the volume of a classroom, and the power of a light bulb;
4. Use past papers and mark schemes: Mark schemes for past papers are usually obtained through schools; check how method and result marks are awarded;
5. Practice math and physics together: Master logarithms, indices, trigonometry, and vectors in math classes to make physics calculations smoother; see Subject Combination: Math + Further Math + Physics.

Conclusion

A large portion of the marks for A-Level Physics calculation questions depends on strict conventions: memorizing all formulas (Cambridge) or knowing how to look them up (Edexcel), converting units to SI, using significant figures according to the rules, writing out the full process for "Show that" questions, and including units in the answer. These requirements are all written in the syllabus. Turning them into habits will help you secure marks steadily in calculation questions. If you need A-Level Physics tutoring, feel free to chat with our teachers.

Verification note: This article was verified in September 2026: which Cambridge 9702 papers include the formula sheet, the constants on the formula sheet (g = 9.81 m s⁻²), examples of formulas requiring "recall and use" that are not on the formula sheet (we verified each against the formula sheet), Chapter 1 requirements for physical quantities and units, significant figure rules and examples, command word definitions, and mathematical requirements, all from Cambridge International's "Physics 9702 syllabus for 2025, 2026 and 2027"; Edexcel IAL Physics does not require memorizing formulas, formulas and constants are provided with the papers, the value of g, and the minimum mark requirements for mathematical skills, all from the Pearson IAL Physics syllabus (3rd edition); Cambridge 9709 Mechanics paper uses g = 10, which was verified in article 182 on this site. The unit homogeneity example was created by us. Keeping one extra significant figure for "Show that", answering steps, common reasons for losing marks, and practice methods are our recommendations based on experience.

References:

• Cambridge International：AS & A Level Physics (9702)
• Pearson：International Advanced Level Physics