WACE ATAR Physics common mistakes
WACE ATAR Physics · Understand the traps and how to avoid them.
When a question asks for a vector quantity (velocity, acceleration, force, momentum), the candidate reports only the magnitude and omits direction or a sign convention, even though the working is otherwise correct.
Candidates habitually treat physics numbers as scalars because most early practice is numeric, and they read the stem for 'how fast' or 'how big' rather than 'what is the velocity'. Under time pressure, the direction feels like an optional extra rather than part of the answer.
What to do instead
Before writing any final answer, classify the requested quantity as scalar or vector and, for vectors, write 'magnitude: ... , direction: ... (relative to stated axis)' as a template. Practise by re-labelling every quantity in past-paper stems as S or V, and deduct from your own practice marks whenever a direction is missing.
The candidate substitutes numbers into a formula without first converting to SI base units, e.g. using grams, centimetres, minutes or kilometres per hour directly in equations expecting kilograms, metres and seconds.
Exam stems often supply values in convenient everyday units, and candidates pattern-match to the formula they recognise without a unit check. Mixed units within one question (e.g. mass in g, speed in km/h) are especially easy to miss.
What to do instead
Adopt a rigid first step: rewrite every given value with its unit, then convert to SI before touching any equation, showing the conversion. Keep a memorised conversion table (1 km/h = 1/3.6 m/s, 1 g = 10^-3 kg, 1 cm = 10^-2 m) and annotate each converted value in your working.
In multi-stage motion problems, the candidate treats the whole journey with one set of kinematics equations instead of splitting it into segments where acceleration is constant, and fails to carry the final velocity of one segment into the next as the initial velocity.
Candidates see one narrative ('a car accelerates then brakes') and reach for a single equation. They also forget that displacement, velocity and time must be accumulated across segments rather than reset.
What to do instead
Draw a velocity-time sketch and mark every change in acceleration as a boundary. For each segment, list the five kinematics variables (u, v, a, t, s), fill in what is known from the previous segment, and solve only within that segment. Total displacement is the sum of segment areas under the v-t graph.
The candidate analyses forces on a single object but misidentifies which forces act on it, typically adding a force that the object exerts on something else, or omitting a contact force such as friction, normal force or tension.
Newton's third-law pairs are conceptually confused with the forces on one body, and candidates reason from intuition ('the engine pushes the car forward') rather than from an explicit free-body diagram.
What to do instead
Always draw a free-body diagram with the object isolated and only forces acting on it, each labelled with its source. Check each force has a partner acting on a different body. Verify the diagram against the object's acceleration direction before writing net force equations.
On inclined-plane and circular-motion questions, the candidate resolves the weight into components but uses the wrong trigonometric function, or forgets that the centripetal force is the net inward force rather than an extra force to be added.
The angle used in the diagram is often measured from the horizontal while the component formulas assume the angle from the vertical, and candidates memorise 'mg sin theta' without checking which component is parallel to the surface. In circular motion, they add centripetal force to tension or gravity instead of equating it to their resultant.
What to do instead
Redraw the triangle at the object, mark the angle explicitly, and derive the components from the geometry each time rather than recalling a formula. For circular motion, write 'net inward force = mv^2/r' and list which real forces contribute to that net force, never adding a separate 'centripetal force' arrow to the diagram.
In energy problems, the candidate equates initial and final mechanical energy without accounting for non-conservative work such as friction, air resistance or a driving force, so the energy bookkeeping is incomplete.
Conservation of energy is learned first as a clean pendulum or roller-coaster case, and candidates apply the same equation to situations where energy is dissipated or added. They also confuse work done by friction with the friction force itself.
What to do instead
Write the general statement: initial mechanical energy + work done by non-conservative forces = final mechanical energy, with signs explicit (friction work negative). Identify each energy store at the start and end (kinetic, gravitational potential, elastic, thermal) and confirm no store is unaccounted for before solving.
The candidate treats momentum as conserved in every collision, including cases where an external impulse acts, or applies kinetic-energy conservation to inelastic collisions.
Momentum and energy conservation are taught together and candidates blur when each applies. They also assume 'collision' implies elastic unless told otherwise.
What to do instead
For every collision, state the system boundary and check that net external impulse is negligible before using momentum conservation. Separately test whether kinetic energy is conserved: if the objects stick together or deformation/heat is mentioned, it is inelastic, so use momentum only and compute the kinetic-energy loss explicitly.
When describing waves, the candidate confuses frequency, wavelength, speed and period, or applies v = f lambda to a situation where the medium changes and assumes frequency changes with speed.
The symbols look interchangeable in routine drills, and candidates do not internalise that frequency is set by the source and remains constant across a boundary while speed and wavelength change.
What to do instead
For any wave-boundary problem, write three lines: frequency fixed by source, speed determined by medium, wavelength = speed/frequency. Practise with a wave moving from deep to shallow water or between media, tabulating f, v and lambda before and after the boundary and checking which quantities are invariant.
In electric and magnetic field questions, the candidate uses the wrong sign for charge or forgets that field direction is defined by the force on a positive test charge, and misapplies the right-hand rule for moving charges or current-carrying wires.
Electron flow versus conventional current is a persistent source of sign errors, and the right-hand rule is memorised as a gesture without a consistent mapping to the coordinate axes drawn in the question.
What to do instead
State the sign convention at the top of every fields question ('conventional current', 'positive test charge'). For right-hand-rule problems, draw the axes, label fingers/thumb or palm explicitly, and check the result against a simple known case (e.g. a positive charge moving east in a field into the page).
On extended-response and practical-analysis questions, the candidate writes a conclusion or evaluation that restates the result without linking it to the hypothesis, the uncertainty, or the reliability of the data, and omits units, uncertainties and significant figures in tables and graphs.
Candidates treat the discussion section as a summary of what happened rather than an evaluation of evidence. They also have not practised attaching absolute or percentage uncertainty to measurements and propagating it to a calculated value.
What to do instead
Use a fixed structure for evaluations: state the claim, cite the specific data or trend supporting it, quantify uncertainty (e.g. +/- 0.5 cm, percentage error), identify one major source of error and one improvement. In tables, include units in headers and uncertainties on every measured value; in calculations, carry uncertainty through to the final result and round to the appropriate significant figures.