HSC Physics common mistakes
HSC Physics · Understand the traps and how to avoid them.
Writing a numerical answer with no unit, or carrying units inconsistently through a multi-step calculation (for example dropping the metre from a projectile result and reporting speed as a bare number).
Under exam pressure candidates treat the arithmetic as the whole task and stop once the number appears, and they often convert to SI units at the start but then silently revert to centimetres or grams mid-solution.
What to do instead
Adopt a fixed habit: write the given quantities with units, convert all to SI, then carry units through every line and box a final answer that includes the unit and direction where relevant. Practise by marking your own past answers only for units before checking the value.
Treating a vector quantity as a scalar, such as adding speeds arithmetically when two velocities are perpendicular, or writing a force magnitude without direction in a free-body answer.
Many earlier calculations in the course involve scalars, so the reflex is to add magnitudes; the vector nature of velocity, acceleration, force and momentum is easy to state but not to apply under time pressure.
What to do instead
Before computing, label each given quantity as vector or scalar. For any vector problem, draw a diagram with an arrow direction for each vector, then resolve into components or use a triangle before combining. Check the answer against the diagram: a resultant should lie between the two inputs in direction.
Confusing mass and weight, or using g as a force rather than an acceleration, for example writing that a 5 kg object 'has a weight of 5 kg' or omitting g when finding a normal force.
Everyday language uses kilograms for weight, and the symbol g is used both for gravitational field strength and for acceleration due to gravity, so the two ideas blur.
What to do instead
State the distinction explicitly in every solution: mass in kilograms is a measure of inertia and does not change with location; weight in newtons is the gravitational force mg. In free-body diagrams write forces in newtons only, and treat g as an acceleration of about 9.8 m/s squared acting downwards.
Applying conservation of energy to a situation where significant work is done by friction or an external force, or conversely assuming mechanical energy is lost when it is actually conserved.
Candidates memorise 'energy is conserved' without tracking the system boundary, so they do not ask whether the chosen system includes the surfaces and the thermal energy they gain.
What to do instead
Before writing an energy equation, define the system and list every energy transfer across its boundary. If friction acts and is outside the system, include a work-done-against-friction term. Write the general statement initial energy plus work done equals final energy, then delete terms that are zero.
Using a kinematics equation for constant acceleration in a situation where acceleration varies, such as a mass on a spring, a charged particle in a non-uniform field, or an object entering a resistive medium.
The constant-acceleration equations are heavily practised and easy to reach for, and the question may mention a familiar setup without explicitly flagging that acceleration is not constant.
What to do instead
Before selecting an equation, write down what is known about the acceleration and justify that it is constant. If it is not, switch to energy methods or to analysing forces at a point. Train this by classifying practice problems into constant-acceleration and non-constant-acceleration categories before solving.
Drawing a free-body diagram with extra forces that are not real, such as a 'force of motion' in the direction of travel, or omitting a force such as friction, tension or the normal force.
Candidates reason from the object's motion rather than from interactions, so they add a force to explain why something moves, and they forget contact forces at surfaces they cannot see.
What to do instead
Draw the object as a dot, then list interactions one at a time: gravitational, each contact surface, each string or rod, each field. Draw only forces exerted on the object by something else, and check that each arrow can be attributed to a specific source. Remember that constant velocity means zero net force, not a forward force.
Describing a wave or field qualitatively when the question asks for a calculation, or giving the formula and then failing to substitute correctly, for example mixing frequency and period or using degrees in a radian formula.
Waves and fields questions often combine a conceptual part with a quantitative part, and candidates answer the part they find easier while running out of time for the other.
What to do instead
Read the question and underline the command word and the quantity requested with its unit. Write the relevant relationship, rearrange it symbolically, then substitute. Keep a one-line unit check: hertz times seconds must give a dimensionless number. Practise mixed conceptual and numerical parts as single questions.
Stating a physics principle by name without applying it to the specific situation, for example writing 'momentum is conserved' in a collision where an external impulse acts, or 'by Newton's third law' without identifying the pair of forces.
Candidates learn principle statements as labels to insert into answers, and the marking criteria reward application rather than naming, so the response looks correct but scores little.
What to do instead
After naming a principle, immediately write the condition under which it holds and check that condition against the scenario. For Newton's third law, name both objects and both forces and state that they act on different bodies. Structure answers as claim, condition, application, conclusion.
Giving a conclusion with no supporting reasoning or evidence, such as answering a 'justify' or 'evaluate' question with a single sentence that restates the result.
Extended-response questions are left until the end, so they are answered in a hurry, and candidates underestimate how many marks come from linking evidence to the judgement.
What to do instead
Use a fixed scaffold for extended responses: state the judgement, give at least two pieces of specific evidence with the physics principle behind each, address a limitation or counter-argument, then restate the judgement in light of the evidence. Rehearse this on past extended-response questions under timed conditions.
Treating a model or theory as a literal description rather than a representation with limits, for example claiming a ray model explains diffraction fully or that a scientific model is simply wrong once it is superseded.
School treatments often present models as facts to be learned, and the nature-of-science strand is studied as content to recall rather than as a way of evaluating claims.
What to do instead
For each model you study, write a short card listing what it explains, what it cannot explain, and the evidence that supports it. In answers, use the pattern 'this model is useful for X because it predicts Y, but it fails to account for Z, which requires a different model'.