VCE Physics common mistakes
VCE Physics · Understand the traps and how to avoid them.
When adding or resolving vectors, candidates add magnitudes directly (e.g. 3 N + 4 N = 7 N) even when the forces act at an angle, instead of using components or the cosine rule.
Students transfer scalar arithmetic habits from earlier maths and do not draw a vector diagram, so they never notice the angle matters; the formula 'a² = b² + c² + 2bc·cosθ' is misapplied with the wrong angle.
What to do instead
Always draw a labelled free-body or vector triangle before calculating, mark the included angle explicitly, and check the answer lies between |a−b| and a+b. Practise resolving into perpendicular components with a sign convention stated at the top of the working.
Candidates state Newton's third-law pairs incorrectly, pairing a force with the same object's weight or with a force on a different body (e.g. 'normal force and weight are an action-reaction pair').
The everyday phrasing 'equal and opposite' hides the requirement that the two forces act on different bodies; students match forces by magnitude rather than by interaction.
What to do instead
For every third-law pair, name both objects and the interaction: 'A on B' and 'B on A'. Write the pair as two full sentences before deciding; if both forces act on the same object, they are a balanced pair, not a third-law pair.
On motion graphs, candidates read a position–time graph as if its slope were acceleration, or treat a curved displacement–time graph as constant velocity, and confuse area under the curve with the wrong quantity.
Graph interpretation is taught quickly and students memorise 'gradient = speed' without linking each graph type to its physical meaning; they also ignore axis labels and units.
What to do instead
Before reading any graph, write the axis quantities and state which derivative or integral gives velocity, acceleration, or displacement. Annotate the graph with tangent lines or shaded areas and check units (m/s from m÷s) to confirm the interpretation.
In circuit problems, candidates apply the parallel-resistance formula to series branches (or vice versa) and assume current divides equally regardless of resistance.
The reciprocal formula is memorised as a string without the underlying idea that parallel branches share the same voltage; students also skip drawing the circuit as a labelled diagram.
What to do instead
Redraw every circuit with nodes labelled and mark which components share a node (same voltage). Compute equivalent resistance branch by branch, then use V=IR at each node to find currents; verify that currents sum correctly at junctions.
Candidates treat the energy stored in a capacitor or spring as linear in the variable (using E = ½QV incorrectly as E = QV, or E = kx instead of ½kx²) and forget the factor of one half.
The area-under-the-graph derivation is often skipped, so the half is memorised inconsistently; students also confuse peak values with average values.
What to do instead
Derive the stored energy from the area of the relevant graph (triangle for a linear spring or capacitor) each time, and write the factor ½ explicitly. Sanity-check with units and with a known simple case before substituting numbers.
On projectile and circular-motion questions, candidates split motion incorrectly-using the horizontal component of velocity in vertical equations, or omitting the centripetal force from the net-force equation at the top of a loop.
Independence of horizontal and vertical motion is stated but not practised; in circular motion students treat 'centripetal force' as an extra force rather than the resultant of real forces.
What to do instead
Set up two columns (x and y) and forbid mixing variables between them; state that time is the only shared quantity. For circular motion, draw the free-body diagram and write ΣF toward the centre = mv²/r, listing only real forces on the left-hand side.
Candidates apply conservation of momentum without accounting for direction, treating momentum as a scalar and ignoring the vector nature in collisions and explosions.
Momentum is introduced alongside scalar energy, and students carry over the habit of just adding numbers; sign conventions are not written down before substitution.
What to do instead
Define a positive direction explicitly at the start, assign signs to every velocity, and write the full vector equation before substituting. Check that the total momentum after equals the total before in both magnitude and sign.
In gravitational and satellite problems, candidates substitute the altitude above the surface for the orbital radius r, or use g = 9.8 m/s² at orbital heights.
Textbook diagrams label 'height h' prominently, and students conflate the surface field strength with the field at distance; they also forget r is measured from the centre of mass.
What to do instead
Always write r = R_planet + h before substituting and state that gravitational force acts from the centre. Use g = GM/r² for the local field and check whether the question is asking for surface or orbital values.
On electromagnetic induction questions, candidates predict an induced current direction that reinforces the change in flux, or claim a current is induced when flux is constant but large.
Lenz's law is memorised as a phrase without applying 'opposes the change'; students focus on the magnitude of flux rather than its rate of change.
What to do instead
Apply a fixed three-step routine: identify the change in flux, determine the direction of the induced field that opposes that change, then use the right-hand rule for the current. State explicitly that a steady flux gives zero EMF.
Candidates answer 'explain' and 'evaluate' questions with a formula and a number only, omitting the physical reasoning, assumptions, and limitations that the marking scheme rewards.
Students practise calculation drills but not extended-response structure, and under time pressure they treat explanation questions as computation questions.
What to do instead
Use a scaffold for extended answers: state the principle, apply it to this situation, give the calculation, then state one assumption or limitation. Practise writing two or three such responses per topic and self-mark against the command word in the question.