Why calculations lose marks
Most marks lost on Physics calculations come from the routine, not the physics. Students jump straight to a number, skip the line where they substitute, or finish with no unit.
Cambridge mark schemes usually split a calculation into method marks and an answer mark. A correct answer will often earn full marks on its own, but if you slip anywhere, only the working on the page can rescue the method marks.
The five-step routine
Use these steps in this order every time.
-
Write down what you know
List every quantity the question gives, with its symbol and unit, and note what you are asked to find.
Tip. Convert units now: cm to m, g to kg, minutes to s, kJ to J. -
Write the equation in symbols
For example P = W ÷ t. If the unknown is not the subject, rearrange the symbols first, before any numbers go in.
-
Substitute, with units
Replace each symbol with its value. Keeping the units in this line catches mismatches such as grams mixed with kilograms.
-
Calculate
Work out the answer and keep the extra digits for now. Rounding halfway through a multi-step problem is how answers drift off.
-
Give the answer with its unit
Round to two or three significant figures, add the unit, then ask whether the size is sensible.
Tip. A car with a mass of 15 kg, or a time of 0.0002 s, should make you stop and check.
If a question has several parts, repeat the routine for each part and carry your earlier answer forward. A wrong value used correctly in the next step can still earn marks, but only if the working is on the page.
Worked example
This question needs three equations in a chain: a force first, then work done, then power.
A crane lifts a load of mass 1 200 kg vertically through 15 m in 20 s. The gravitational field strength is 10 N/kg. Calculate the useful power output of the crane.
Notice the first line. The load has a mass in kilograms, but the lifting force is a weight in newtons. Skipping that line is the most common way to lose these marks.
Equations used here
Two meanings of W. The same letter is used for weight and for work done. Write the word next to the symbol in your working so the examiner can follow it.
A student of mass 50 kg runs up a flight of stairs 4.0 m high in 8.0 s. Take g = 10 N/kg. Calculate the useful power she develops.
Show the answer
Work done = 500 × 4.0 = 2 000 J
P = 2 000 ÷ 8.0 = 250 W