Calculation questions account for a significant proportion of marks on both Paper 2 and Paper 4. The method is consistent across all calculation types.
The 4-step method
Every calculation question follows the same structure:
Step 1: Write the equation. State the equation you are going to use. This earns a mark even if you make an arithmetic error later.
Step 2: Substitute the values. Put the numbers from the question into the equation. Show this substitution clearly. If a value needs converting (e.g., km to m or minutes to seconds), do the conversion before substituting.
Step 3: Calculate the answer. Do the arithmetic. Show intermediate steps if the calculation has multiple stages.
Step 4: Write the answer with the correct unit. The unit is often worth a separate mark. Check that the unit matches the quantity you calculated.
Common calculation types
Speed, distance and time
- s = d / t (or d = s x t, or t = d / s)
- Always check: is distance in m or km? Is time in s or hours?
Force, mass and acceleration
- F = m a
- Weight: W = m g (g = 10 N/kg or 9.8 N/kg, check the paper)
Work, force and distance
- W = F d
- Ensure force and distance are in the same direction
Power
- P = W / t (or P = E / t)
- Also P = I V for electrical power
Resistance
- V = I R
- Rearrange for I or R as needed
Energy
- KE = 0.5 m v squared
- GPE = m g h
- Thermal energy: E = m c delta-theta
Pressure
- p = F / A (solid surfaces)
- p = rho g h (liquids)
Practice strategy
For each equation type above, find 3 past paper questions. Work through them using the 4-step method. After each one, check the mark scheme to see if you earned every mark.
Where marks are lost
- Not writing the equation. This mark is effectively free.
- Substituting before converting units. Always convert to SI units first.
- Missing the unit in the final answer. Write it.
- Rounding errors. Follow the precision given in the question. If the question gives values to 2 significant figures, give your answer to 2 or 3 significant figures.
- Not showing working. If you jump straight to the answer and it is wrong, you earn zero. If you show working, you can earn method marks even with an arithmetic error.
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