Common Calculation Errors in IGCSE Physics

Certain calculation errors appear repeatedly in IGCSE Physics papers. Knowing them helps you check your work before moving on.

Error 1: Wrong unit conversion

Problem: Using cm instead of m in a formula that requires SI units. Example: Calculating pressure with area = 50 cm^2 but not converting to m^2. Fix: Convert 50 cm^2 to 50 x 10^-4 m^2 = 5.0 x 10^-3 m^2 before substituting.

Error 2: Forgetting to square or square root

Problem: Kinetic energy = 0.5 x m x v, forgetting to square v. Correct: KE = 0.5 x m x v^2. Fix: Write the formula first, then substitute. The formula reminds you.

Error 3: Dividing instead of multiplying (or vice versa)

Problem: Using F = m / a instead of F = m x a. Fix: Write the formula, rearrange if needed, then substitute.

Error 4: Wrong standard form entry

Problem: Entering 3.2 x 10^8 as 3.2 x 10 x 8 on the calculator, getting 256. Fix: Use the EXP button: 3.2 EXP 8.

Error 5: Premature rounding

Problem: Rounding an intermediate answer to 1 s.f. and using that in the next step, introducing a large error. Fix: Keep full calculator values until the final answer.

Error 6: Missing the unit

Problem: Writing “75” instead of “75 J”. The unit mark is separate and easy to earn. Fix: Always write the unit. Check it matches the quantity.

Error 7: Using the wrong formula

Problem: Using the wave speed formula (v = f x lambda) when the question asks about the speed of an object (v = d / t). Fix: Read the question carefully. Identify which physics concept applies.

Self-check routine

After every calculation:

1. Does the formula match the situation?
2. Are all values in SI units?
3. Is the substitution correct?
4. Does the answer have a reasonable size?
5. Is the unit correct?