AEPA Physics (NT308) common mistakes
AEPA Physics (NT308) · Understand the traps and how to avoid them.
Solving projectile or incline problems by plugging numbers into a memorized formula without first drawing a free-body diagram and choosing axes, so sign errors and omitted components go unnoticed.
Candidates memorize formulas like the range equation or a = g sinθ but skip the setup step under time pressure, losing track of which direction is positive and which forces act.
What to do instead
For every mechanics problem, force the habit of a 20-second sketch: draw the object, all forces, and label the positive axis direction before touching the calculator. Check that the number of forces matches the physical situation.
Confusing mass with weight and treating g as a variable to be solved for, or reporting weight in kilograms.
Everyday language blurs mass and weight, and candidates carry that into problems involving apparent weight, normal force in elevators, or orbital motion.
What to do instead
Write units on every intermediate quantity. If the answer's unit is N, the quantity is a force; if kg, it is mass. Practice elevator and scale problems until the distinction is automatic.
Applying conservation of mechanical energy to systems with friction or applied non-conservative forces without an energy-input term.
Candidates see a height and a speed and reflexively set mgh = ½mv², forgetting that friction removes energy or that a push adds it.
What to do instead
Before writing an energy equation, list every force doing work and classify it conservative or not. Include a W_nc term explicitly, then set it to zero only when justified.
Mixing up series and parallel rules for resistors and capacitors, especially using 1/R_total = sum(1/R) for parallel resistors but then applying the same reciprocal logic to capacitors.
The two sets of rules are reciprocal to each other, and candidates learn them close together and overwrite one with the other under stress.
What to do instead
Anchor the rules physically: parallel resistors share voltage and offer more paths (total resistance drops); series capacitors share charge and store less (total capacitance drops). Rebuild the rule from the physical statement rather than recalling the formula.
Treating current as something that is used up as it passes through a series circuit, or assuming the current before a resistor is larger than after it.
The water-flow analogy is often overextended, leading candidates to think resistors consume current rather than dissipate energy via voltage drop.
What to do instead
State the junction rule aloud for any circuit: charge in equals charge out, so current is identical at every point in a series branch. Only voltage drops across components.
Assuming a wave's frequency changes when it moves from one medium to another, or when it reflects off a boundary.
Candidates conflate frequency with speed because they change together in everyday experience, and they forget that frequency is set by the source.
What to do instead
Memorize the invariant: frequency is fixed by the source and is unchanged by medium changes or reflection; speed and wavelength adjust together. Use v = fλ to check which quantity must change.
Applying the mirror/lens equation with a sign convention chosen inconsistently mid-problem, producing a virtual image reported as real or a negative focal length misinterpreted.
Different textbooks use different sign conventions, and candidates blend them, especially for diverging lenses and convex mirrors.
What to do instead
Pick one convention at the start of each problem (e.g., real-is-positive), write it at the top of the scratch work, and verify the result against a quick ray diagram for object type and image orientation.
Confusing heat with temperature, or assuming that adding heat always raises temperature.
Everyday usage treats the words as synonyms, and candidates forget phase changes, where energy goes into breaking bonds at constant temperature.
What to do instead
Define heat as energy transferred and temperature as average kinetic energy. When a problem involves melting or boiling, check for a latent-heat plateau before applying Q = mcΔT.
Applying the second law of thermodynamics backwards, claiming that a process with positive entropy change cannot occur or that heat flows spontaneously from cold to hot.
Candidates memorize 'entropy always increases' without connecting it to the direction of spontaneous processes, then misapply the inequality.
What to do instead
Rehearse the statement: the total entropy of an isolated system never decreases, so ΔS_universe ≥ 0 for any allowed process. Test each proposed process by computing ΔS_system + ΔS_surroundings, not the system alone.
Treating the photoelectric effect as requiring brighter light rather than higher-frequency light to eject electrons, or mixing up which variable controls current versus stopping voltage.
Classical wave intuition suggests intensity should supply the energy, and candidates have not fully replaced that intuition with the photon model.
What to do instead
Write the photoelectric equation explicitly, hf = φ + K_max, and label each variable's experimental control: frequency sets K_max and stopping voltage; intensity sets the number of electrons (current) once above threshold.