VCE Physics: units and significant figures that cost marks

In VCE Physics, some of the most frequently dropped marks have nothing to do with understanding the physics. They are lost on units, significant figures, and missing directions — presentation, not knowledge. Because short-answer and extended questions award method marks for the setup and working, getting disciplined about these details is one of the fastest ways to lift a Physics score.

Units: carry them through every line

Write the relationship you are using, substitute values with their units, then evaluate. Carrying units through is not just neatness — it catches errors (if the units don't resolve to newtons, your working is wrong) and it secures the answer mark, which is often explicitly for a correctly-stated quantity with units. A bare number, even a correct one, can lose that mark.

Significant figures: match the data

Quote your final answer to a number of significant figures consistent with the data in the question — usually two or three. Writing six figures straight off a calculator signals you haven't thought about precision, and rounding too early in the working introduces errors that shift the final answer. Keep full precision in intermediate steps and round only at the end.

A worked example: right number, lost mark

Question (3 marks): A 1200 kg car rounds a flat circular bend of radius 50 m at a constant 15 m/s. Calculate the net force on the car and state its direction.

A student writes: F = mv²/r = 1200 × 15² / 50 = 5400 N.

Marking (illustrative, not an official VCAA scheme): using the centripetal relation earns a method mark, and the correct magnitude of 5400 N earns the answer mark. The third mark is lost — the question said 'state its direction', and the net (centripetal) force points horizontally toward the centre of the bend. Force is a vector; when a question asks for direction, a number alone cannot score full marks.

The fix is a habit: before moving on, check you've answered everything the question asked, with units, to sensible significant figures, and with a direction when the quantity is a vector.

A 15-second check before you move on

• •Units carried through and correct on the final answer?
• •Significant figures match the data (not six off the calculator)?
• •Direction stated for any vector quantity the question asks about?
• •Did I round only at the end, not mid-working?