OSAT Physics (OK114) common mistakes
OSAT Physics (OK114) · Understand the traps and how to avoid them.
When a block slides down a ramp or a car rounds a banked curve, the candidate sets the normal force equal to mg (or mg cos θ plus or minus a friction term) without drawing the free-body diagram in the tilted frame, then solves for acceleration using that wrong N.
Introductory courses drill N = mg on level ground, and candidates memorize the ramp result N = mg cos θ without re-deriving it, so they misplace the cosine when a horizontal applied force, a banked surface, or a vertical loop is involved. The error is silent because the resulting number looks plausible.
What to do instead
Before any algebra, draw the free-body diagram with axes along and perpendicular to the surface, resolve each force into those axes, and write ΣF_perp = ma_perp. On a banked curve with no friction, show explicitly that N cos θ = mg and N sin θ = mv²/r. Practice three cases: flat curve with friction, frictionless banked curve, and banked curve with friction, and state which component balances weight in each.
The candidate treats mechanical energy as conserved in a system where friction, air resistance, or an inelastic collision occurs, writing ½mv² = mgh for a block sliding down a rough incline or for a ballistic pendulum, instead of accounting for the lost energy.
Energy conservation is taught first in the frictionless case, and candidates pattern-match on the shape of the problem (height given, speed asked) rather than auditing the forces. Inelastic collisions are especially confusing because momentum is conserved while kinetic energy is not.
What to do instead
Adopt a two-step audit: list every force doing work, then classify each as conservative or non-conservative. If any non-conservative force acts, write W_nc = ΔE rather than ΔE = 0. For collisions, decide first whether the collision is elastic, inelastic, or perfectly inelastic, then use momentum conservation always and energy conservation only when elastic. Work one ballistic-pendulum problem end to end, showing that momentum conservation governs the collision and energy conservation governs the swing.
The candidate confuses the direction of induced current or induced magnetic field by applying the right-hand rule to the motion of the conductor rather than to the change in magnetic flux, and often reverses Lenz's law, predicting an induced field that aids the change instead of opposing it.
The right-hand rules for current, force, and field are taught separately and candidates collapse them into one rule. Lenz's law requires reasoning about the change in flux, which is a second-order step that is easy to skip under time pressure.
What to do instead
Use a fixed procedure: identify the original B field direction, determine whether flux is increasing or decreasing, then choose the induced B field that opposes that change, then use the right-hand rule to get the induced current. Test each answer against energy conservation: if the induced current aided the change, the system would generate energy for free. Practice with a magnet falling through a loop, a loop entering and leaving a field region, and a rotating coil.
The candidate computes a thin-lens or mirror image using 1/f = 1/d_o + 1/d_i with signs chosen by memory, producing a virtual image where a real one exists (or vice versa), and then reports the wrong magnification sign or orientation.
Sign conventions differ between textbooks and between lenses and mirrors, and candidates memorize a table without a physical model of where the rays converge. The reciprocal arithmetic also invites errors when d_o is close to f.
What to do instead
Anchor on the physics: a real image forms where rays actually converge and can be projected on a screen; a virtual image forms where backward extensions meet. Draw the three principal rays for every problem before computing. Use the Cartesian convention consistently (distances positive on the outgoing side), and check the answer by asking whether the image should be on the same side as the object for a mirror or the opposite side for a lens. Verify with a limiting case, such as an object at 2f giving an image at 2f with magnification −1.
In a thermodynamics problem, the candidate applies the ideal gas law or the first law using temperature in degrees Celsius, or computes an efficiency or a heat transfer without converting to kelvin, and in kinetic-theory items equates average speed with rms speed.
Classroom problems often quote temperatures in Celsius, and the conversion becomes automatic only after repeated practice. The distinction between v_rms, v_avg, and v_most probable is a subtle statistical result that is easily blurred.
What to do instead
Write T in kelvin as the first line of every gas-law and thermodynamics solution, before substituting anything. Memorize the ratios: v_rms = √(3kT/m), v_avg = √(8kT/πm), v_mp = √(2kT/m), so v_rms > v_avg > v_mp. For efficiency, always compute η = 1 − T_c/T_h with both temperatures in kelvin and sanity-check that η lies between 0 and 1.
The candidate answers a projectile or circular-motion question by applying a kinematic equation at a point where acceleration is not constant, or neglects that the horizontal and vertical motions of a projectile are independent, mixing them into a single equation.
The constant-acceleration equations are the most rehearsed tool in mechanics, so candidates reach for them even when only gravity acts vertically and horizontal velocity is constant. Circular motion adds a centripetal acceleration that changes direction continuously, which the equations cannot handle.
What to do instead
Separate projectile problems into two columns, horizontal (a = 0) and vertical (a = −g), and solve each independently, linking them only through time. For circular motion, use only the centripetal relation a_c = v²/r and tangential kinematics, and never apply v = v₀ + at to the changing direction of velocity. When a problem involves both, such as a projectile launched from a rotating platform, state which equation applies to which component before substituting.
The candidate reports a calculated result with more significant figures than the given data supports, or drops units mid-calculation and then supplies an incorrect unit (for example, reporting energy in newtons or frequency in seconds) on the final answer.
Calculators return many digits by default, and candidates treat the display as the answer. Unit tracking is often skipped because the numerical formula feels familiar, so dimensional errors go undetected.
What to do instead
Carry units through every step as algebraic quantities and cancel them explicitly; if the final unit is not the one requested, the setup is wrong. Apply the multiplication/division rule (result limited by the factor with fewest significant figures) and the addition rule (limited by fewest decimal places). As a final check, estimate the answer to one significant figure mentally and confirm the calculator result is the same order of magnitude.
The candidate confuses the photoelectric effect with Compton scattering or with pair production, and misstates which observation (threshold frequency, instantaneous emission, or wavelength shift) supports the photon model, or reverses the relationship between photon energy and wavelength.
These three phenomena all involve photons and electrons, and exam items test the specific evidence each provides. Candidates also memorize E = hf without internalizing that E and λ are inversely related through c = fλ.
What to do instead
Build a comparison table with columns for phenomenon, what is incident, what is emitted, and the key observation: photoelectric (threshold frequency and instantaneous emission prove quantization), Compton (wavelength shift proves photon momentum), pair production (energy-to-mass conversion near a nucleus). Drill the relation E = hc/λ in both directions, and confirm that a shorter wavelength means a higher-energy photon.
The candidate sets up a circuit with series and parallel branches and adds resistances directly across the whole network, or applies the parallel voltage rule to a series branch, without reducing the network step by step or checking Kirchhoff's junction and loop conditions.
The series and parallel rules are learned as formulas rather than as consequences of charge and energy conservation, so candidates misclassify branches when a network is not drawn in the textbook's canonical shape.
What to do instead
Redraw the circuit in a standard layout, labeling nodes, then reduce from the innermost parallel group outward, replacing each group with its equivalent resistance. Verify with Kirchhoff's laws: the sum of currents at every junction must be zero and the sum of voltage changes around every loop must be zero. Check the result physically: the equivalent resistance of a parallel combination must be smaller than the smallest branch resistance.
The candidate designs or evaluates a lab procedure without identifying the controlled variables, the independent and dependent variables, or the sources of systematic versus random error, and then proposes a fix that addresses precision rather than the actual bias.
Experimental design is often studied as vocabulary rather than as a decision procedure, so candidates list 'human error' generically instead of naming a specific mechanism such as parallax, zero offset, or reaction time.
What to do instead
For every procedure, write four lines before evaluating it: independent variable, dependent variable, controlled variables, and the measurement that yields the result. Then classify each error as random (reduces with averaging, affects precision) or systematic (shifts all readings, affects accuracy) and match the remedy accordingly, such as calibrating an instrument for a zero offset versus repeating trials for scatter. When reading a graph, state what the slope and intercept physically represent and whether they match the predicted relationship.