SACE Stage 2 Physics common mistakes
SACE Stage 2 Physics · Understand the traps and how to avoid them.
Using the total time of a journey as the time interval in v = Δs/Δt when the motion is not uniform, or substituting a velocity value into the acceleration formula instead of a velocity change.
Students memorise the formulas as templates and pick values by keyword matching ('time', 'speed') rather than by asking which two events define the interval. The SUVAT symbols look interchangeable, so Δv collapses to v and Δs collapses to s.
What to do instead
Before substituting, write a data line with the two events named (e.g. 'from t = 0 at release to t = 3.0 s at impact') and list only quantities measured between those events. Cross out any symbol you have not defined. Practise by classifying ten short scenarios as uniform or non-uniform before calculating anything.
Treating the horizontal and vertical components of projectile motion as sharing the same time, speed, or acceleration, for example applying the vertical acceleration to the horizontal component or using the launch speed in the vertical equation.
The single launch velocity feels like one quantity, and students are used to one acceleration per problem. The independence of the components is stated in class but rarely practised as a separation step, so the components get recombined too early.
What to do instead
Always draw two separate columns headed 'horizontal (a = 0)' and 'vertical (a = g)' and fill each only with its own quantities. Solve the vertical column for time first, then carry that time across as the only shared quantity. Check by confirming the horizontal speed never changes.
Confusing the reference frame in relativity problems: applying the time-dilation formula with the proper time measured in the wrong frame, or treating the dilated time as the proper time when the clock is moving.
The phrase 'proper time' sounds like 'correct time', so students assume it belongs to the observer who is 'really' stationary. Both frames can be treated as stationary, so the label feels arbitrary and gets assigned by habit rather than by which clock measures the two events at the same place.
What to do instead
Identify the two events first, then ask which single clock is present at both events; that clock reads the proper (shortest) time. Write 'events: __ and __; co-moving clock: __' before choosing the formula. Verify with the invariant interval or by checking that the calculated time is longer than the proper time.
Mixing up mass and weight when applying Newton's second law, or adding forces that act on different bodies when analysing connected objects, so the normal force or tension is set equal to mg regardless of acceleration.
Everyday language uses 'weight' in kilograms, and free-body diagrams are often drawn from memory as a single blob with arrows rather than as a careful inventory of forces on one chosen object. Newton's third-law pairs then get cancelled within the same diagram.
What to do instead
Draw one diagram per body, listing only forces acting on that body, and label each with its source. Write the normal force as an unknown and solve for it from the vertical equation, then compare with mg as a check. State explicitly which body each equation belongs to above the working.
Applying Ohm's law to a whole circuit containing more than one resistor, or assuming current is used up as it passes through components, so series and parallel branches are given the same current or the same voltage.
The simple V = IR triangle is over-learnt in single-resistor contexts, and the analogy of current 'flowing' encourages a mental model of a consumable fluid. Kirchhoff's rules are known as statements but not used as a working tool.
What to do instead
Redraw the circuit marking junctions, then apply the junction rule (current in equals current out) and loop rule explicitly before any arithmetic. Treat V = IR as valid only across one identified component. Check the answer by confirming the sum of voltage drops round each loop equals the supply emf.
Determining the direction of the magnetic force on a moving charge or current-carrying wire with the wrong hand rule or the wrong finger assignment, and forgetting to reverse the force for a negative charge.
Several hand rules circulate (right-hand slap, Fleming's left hand, right-hand grip) and they assign different fingers to field, current, and force. Students recall a rule visually but not which quantity each finger represents, and the sign of the charge is easy to drop.
What to do instead
Commit to one rule and write its finger mapping beside every problem (e.g. fingers = field, thumb = conventional current, palm = force). For electrons, find the force as if positive and then reverse it, stating this step in writing. Sanity-check with a simple case whose answer you know, such as a charge moving parallel to the field experiencing zero force.
Using the wrong sign convention in the wave equation or lens/mirror formula, or treating a virtual image distance as positive without checking, leading to an image described as real when it is virtual.
The formula is remembered as a string of symbols, and the sign rules are taught separately from the algebra, so a negative answer is reported as an error rather than interpreted. Diagrams are often skipped because the numbers seem to make them unnecessary.
What to do instead
Write the sign convention at the top of the page before substituting (which side is positive, which focal lengths are negative for diverging elements). After solving, draw a quick ray diagram and check whether the rays actually converge. Interpret the sign in words in the final answer rather than leaving a bare number.
Applying the diffraction or interference condition without matching it to the geometry: using d sin θ = nλ with d as a slit width rather than a slit separation, or applying the single-slit minimum condition to a double-slit pattern.
The same symbols d, θ, and n appear in both the double-slit and single-slit formulas, and the two situations are often drawn identically in notes. Students match on the presence of 'slits' rather than on what is being measured.
What to do instead
Before calculating, write a one-line description of the physical setup (two narrow sources separated by d, or one wide aperture of width w) and choose the formula that names those quantities. Keep a comparison table of the two conditions. Check the order of magnitude of the predicted fringe spacing against a sketch.
Mixing up the photoelectric equation terms, for example treating work function as a frequency or using the threshold frequency in place of the incident frequency when finding maximum kinetic energy.
The equation is a short energy balance, but the three quantities (photon energy, work function, kinetic energy) are all energies in the same unit, so students substitute whichever value appears in the question. Threshold and incident frequency are both described as 'the frequency'.
What to do instead
Write the equation in words first: photon energy minus escape energy equals maximum kinetic energy. Label each given value with its name before substituting, and treat threshold values as a boundary condition to test rather than a number to plug in. Confirm that the answer is zero when the incident frequency equals the threshold.
Balancing nuclear equations by mass number only, or ignoring conservation of charge and lepton number when writing beta decay, so the atomic number is left unchanged or the antineutrino is omitted.
Alpha decay is learnt as a pattern and then generalised, and beta decay is remembered as 'a neutron turns into a proton' without the emitted particles being tracked. Mass number balance is easy to check, so charge balance gets skipped.
What to do instead
Check both mass number and atomic number on each side for every equation, writing the sums underneath. For beta decay, write the full equation including the electron and antineutrino and verify charge and lepton number. Practise by classifying a set of decay equations as correct or incorrect and stating which conservation law each one breaks.