NCEA Level 2 Physics common mistakes
NCEA Level 2 Physics · Understand the traps and how to avoid them.
At the highest point of a projectile's path, writing "acceleration is zero" or "forces are balanced" because the vertical velocity is momentarily zero, and then combining that with an inconsistent sign for g in the working above and below the peak.
Velocity and its rate of change get fused into one idea; the momentary zero feels like "nothing is happening", and candidates memorise "v = 0 at the top" without linking it to a force that never switches off. Sign errors come from choosing downhill-positive in one line and uphill-positive in the next.
What to do instead
State the two facts separately every time: at the top v_vertical = 0 but a = 9.8 m s^-2 downward throughout, because gravity is the only force (ignoring air resistance). Define the positive direction once, on the diagram, and keep it for both the rise and the fall. Sketch velocity and acceleration arrows at three points - rising, peak, falling - with the acceleration arrow unchanged in length and direction.
Writing "E_k at the bottom = E_p at the top" for a ramp, slide or roller-coaster problem that involves friction, getting an answer that is too large, or subtracting the friction work twice by also reducing the height used.
Conservation of energy is learned as a two-state equation and the friction term is treated as optional decoration. Many candidates also never connect "work done against friction = F x d" with the thermal energy appearing in the surfaces, so they cannot see where the missing energy should go.
What to do instead
Always write the full budget line before substituting: E_p(initial) = E_k(final) + W_friction, with W = F_friction x d measured along the slope. On an incline get the friction force from F = mu x N with N = mg cos(theta). As a check, the answer must be smaller than the frictionless case, and the "lost" energy should be quoted as heat in the surfaces.
Drawing a separate arrow labelled "centripetal force" alongside the tension, gravity or friction in a free-body diagram, adding an outward "centrifugal force" so the forces balance, or claiming the net force is zero because the speed is constant.
The phrase "centripetal force" sounds like a distinct kind of force on a par with weight or tension, and the outward sensation when you are the moving object feels physically real. Constant speed is also read as constant velocity, which would indeed mean zero net force.
What to do instead
Name only the real forces acting (tension, weight, friction, normal force), identify which ones point towards the centre, then write "the net inward force provides the centripetal force" and set F_net = mv^2/r. Never draw an arrow labelled "centripetal force" in addition to the physical forces. State explicitly that the direction of motion changes, so the velocity is not constant even at steady speed.
Using the battery's stated emf as the voltage across the external circuit in every calculation, so the predicted current or bulb power comes out too high, and not explaining why a voltmeter across the cell reads less than the emf when the switch is closed.
Early circuit work uses ideal cells, and candidates come to see voltage as a fixed property of the battery rather than something shared between the internal resistance and the external load. The internal resistance is often visualised as a tiny separate component that can be ignored.
What to do instead
Write epsilon = I(R + r) and V_terminal = epsilon - Ir at the top of every circuit question, and check whether the question mentions internal resistance before setting r = 0. Test with limiting cases: as R approaches 0, I approaches epsilon/r, not infinity, and as R becomes very large, V_terminal approaches epsilon. Quote the voltmeter reading as the terminal voltage, not the emf.
Adding parallel resistors with R_total = R1 + R2, or explaining a series circuit by saying "current gets used up" so that the current after a bulb is smaller than the current before it.
The series rule is learned first and then pattern-matched to any circuit diagram. The "used up" idea comes from everyday talk about electricity being consumed, and the fact that the current is identical everywhere in a series branch is genuinely counter-intuitive.
What to do instead
Anchor on conservation of charge: coulombs per second entering a component must leave it, so current is unchanged through components in series and it is the voltage that is shared. For parallel branches, use 1/R = 1/R1 + 1/R2 (or product over sum for two) and check that the total is smaller than the smallest resistor. Confirm with a quick brightness or total-current sanity check.
Saying a current is induced when a magnet is held stationary inside or next to a coil, and getting the induced current direction by guessing ("the magnet attracts it") rather than from the change in flux.
The right-hand grip and slap rules get applied mechanically without first asking what is changing. "Opposes the change in flux" is memorised as a phrase rather than used as an instruction, so the words never get converted into a direction.
What to do instead
Use a fixed three-step routine: (1) is the flux increasing or decreasing, and in which direction is the field? (2) the induced field must oppose that change; (3) apply the right-hand grip rule to convert field direction into current direction. State the stationary case explicitly: no change in flux, therefore no induced emf and no current, no matter how strong the magnet.
Claiming that when light or sound passes into a denser medium its frequency decreases (or that its speed stays the same), then substituting that changed frequency into v = f x lambda.
The equation links three variables and candidates alter whichever one the question names first. There is often no clear model that the frequency is set by the source while the speed is set by the medium, so the roles feel interchangeable.
What to do instead
Fix the principle in one sentence: frequency is determined by the source and does not change on refraction; speed is set by the medium, so the wavelength must change and lambda is proportional to v. Use n = c/v to get the new speed, then find the new wavelength, and check that it is shorter in the denser, slower medium while f is unchanged.
Explaining a nodal line or a quiet spot as "the two waves cancel out" or "they arrive out of phase" without quoting any path difference in wavelengths, and treating a point halfway between two speakers as automatically loud.
Cancellation is treated as the cause rather than the consequence. "Out of phase" is a label rather than a measured quantity, so the explanatory marks for linking phase to a distance difference in wavelengths are never reached.
What to do instead
Always quantify before explaining: at a node the path difference is (n + 0.5) x lambda; at an antinode it is n x lambda, with n = 0, 1, 2... Calculate the path difference from the geometry given, convert it to a number of wavelengths, then write the full sentence "the waves arrive in antiphase because the path difference is half a wavelength, so they superpose destructively and the amplitudes subtract".
Writing a beta-minus decay equation without the emitted electron on the right-hand side, forgetting the antineutrino, or making a bookkeeping slip such as giving alpha decay a change of -2 in mass number.
The symbol manipulation is practised separately from the meaning of the decay, and candidates learn "alpha = helium nucleus" as a fact rather than as a statement about nucleon and charge conservation. The beta electron is easy to drop because the nucleus's own identity barely seems to change.
What to do instead
For every decay, write the mass numbers (A) as a top row and the atomic numbers (Z) as a bottom row, and check both totals balance across the arrow before moving on. Memorise the effect on the nucleus: alpha gives A - 4 and Z - 2; beta-minus gives A unchanged and Z + 1, because a neutron becomes a proton plus an electron; gamma leaves A and Z unchanged but releases energy.
Treating radioactive decay as linear: computing "after three half-lives one third of the sample is left", or interpolating proportionally for a time that is not a whole number of half-lives.
Linear rate reasoning is carried over from speed and constant-rate problems, and the word "half-life" invites the reading "halve it once per half-life, so three half-lives means divide by three". Exponential behaviour has not been seen enough to override that default.
What to do instead
Work in half-lives: remaining activity or nuclei = original x (1/2)^n, with n = t / t_half. Count the halvings explicitly (1/2, 1/4, 1/8...) before substituting, and if n is not a whole number use the exponent form rather than interpolating. State that the count rate halves in equal time intervals, that the decay is random so predictions are statistical, and that the activity approaches but never reaches zero.