OAE Physics (035) common mistakes
OAE Physics (035) · Understand the traps and how to avoid them.
When a problem states a value like 9.8 m/s², 3.0 m, or 2.0 s, the candidate substitutes it and reports an answer to five or six significant figures, or drops the trailing zero in 2.0 and treats it as exact.
High school and undergraduate coursework often rewards 'more digits = more precise,' and candidates carry that habit in. They also forget that measured givens carry uncertainty, so the answer's precision is limited by the least precise given, not by the calculator display.
What to do instead
Before computing, write the number of significant figures in each given next to it. After computing, round the final answer to the smallest count and keep one extra digit only in intermediate steps. Practice with items where the distractors differ solely in digit count, so the habit is tied to a scoring consequence.
On a question describing an experiment, the candidate picks the conclusion that matches the expected result rather than the one the described data support, or accepts a causal claim from a correlational design.
Candidates read the stem as asking 'what should happen' and import textbook expectations. They also conflate 'the variables moved together' with 'one caused the other,' a distinction that is tested frequently in Nature of Science items.
What to do instead
Restate the stem as a question about the evidence: 'Given only these observations, what is supported?' Eliminate any choice containing causal language (causes, produces, due to) unless the design manipulated the variable and controlled alternatives. Flag choices that merely restate the hypothesis as a conclusion.
The candidate solves a Newton's-law problem without drawing a free-body diagram, then either omits a force (often normal force on an incline, or tension in a connected system) or assigns the wrong direction to friction.
Under time pressure, candidates jump to equations because the scenario looks familiar. Inclined planes and multi-body systems are where omitted or misdirected forces most often survive to the final answer, since the algebra still 'works.'
What to do instead
Make a free-body diagram mandatory for every force item, with axes chosen along the acceleration direction. For inclines, resolve weight into components explicitly. For connected bodies, draw each body separately before writing equations, and check that every force has a Newton's-third-law partner somewhere in the system.
The candidate treats kinetic and static friction as interchangeable and uses f = μN to find the maximum static friction, then reports that value as the actual friction force when the applied force is smaller.
The formula looks universal, and textbooks often present μ_s and μ_k side by side without stressing that static friction is a response variable bounded above by μ_sN.
What to do instead
Adopt the rule: static friction equals the applied force up to the maximum μ_sN and only reaches that maximum at the verge of slipping; kinetic friction equals μ_kN once sliding. Before substituting, ask 'is it moving?' If not, solve for the required friction from equilibrium and compare it to the maximum.
The candidate computes power dissipated in a circuit by adding resistor values and dividing voltage by the sum, or uses the total current through a parallel branch, instead of finding the current or voltage specific to that element.
Series-circuit habits persist, and candidates memorize P = IV without tracking which I and which V belong to the element in question. Parallel circuits punish this because voltage is shared, not current.
What to do instead
For every power question, first label each element with its own current and voltage. Reduce the network to an equivalent resistance, find the total current, then work backward branch by branch. Verify with the alternative form P = V²/R, which exposes an error immediately if the wrong V was used.
The candidate predicts the direction of the magnetic force on a moving charge or current-carrying wire using the wrong hand, or forgets to reverse the result for a negative charge.
Right-hand rules are taught in several inconsistent versions (flat hand, curled fingers, cross-product), and candidates mix them. The sign of the charge is a one-word detail that is easy to skim past.
What to do instead
Commit to one rule and rehearse it: for F = qv × B, point fingers along v, curl toward B, and the thumb gives F for a positive charge; reverse for electrons. Do three drill problems per session with electrons and protons mixed, and check the answer against the requirement that F is perpendicular to both v and B.
The candidate applies the thin-lens or mirror equation with a sign convention learned in a different textbook, producing an image distance with the wrong sign, or treats a virtual image as if it could be projected on a screen.
Sign conventions genuinely differ across curricula, and the exam expects one consistent system. Candidates also memorize 'real images are inverted' without connecting it to whether the rays actually converge.
What to do instead
Write the convention at the top of scratch work (for example, real object distance positive, real image distance positive on the opposite side for lenses). After solving, sanity-check with a ray diagram sketch: converging outgoing rays mean a real image; diverging rays that only appear to meet mean a virtual one.
The candidate computes an efficiency or heat-engine problem using temperatures in degrees Celsius in a ratio, or applies the Carnot efficiency to a real engine without noting the irreversibility.
Temperature appears so often in Celsius that the conversion becomes automatic, and ratios of Celsius values look plausible because the numbers are positive and comparable.
What to do instead
Convert every temperature to kelvin before any ratio, efficiency, or ideal-gas calculation, and write 'K' on the line. For engine items, check whether the stem specifies an ideal (Carnot) engine; if not, Carnot efficiency is an upper bound, not the actual value.
The candidate computes a photoelectric-effect threshold or photon energy using the wavelength in nanometers directly with h and c, yielding an energy off by a factor of 10⁹.
Constants are memorized in SI units while wavelengths are given in nm or Å, and the unit mismatch is invisible in the arithmetic because the numbers are small and tidy.
What to do instead
Convert wavelength to meters before substituting, or use the shortcut E(eV) ≈ 1240/λ(nm), which is dimensionally self-checking. After computing, compare the result to a familiar anchor: visible photons are roughly 2–3 eV, so an answer of 10⁻¹⁸ eV signals a unit slip.
The candidate answers a half-life or radioactive-decay item by halving the initial amount once per elapsed time rather than once per half-life, or misreads 'fraction remaining' as 'fraction decayed.'
The word 'half-life' primes a single halving, and the distinction between remaining and decayed is a single word in the stem that is easy to miss under time pressure.
What to do instead
Write the exponent explicitly: N = N₀(1/2)^(t/T), and compute t/T as a number before doing anything else. Underline 'remaining' or 'decayed' in the stem and, if decayed is asked, subtract the remaining fraction from one. Check that the answer lies between the initial amount and zero.