GTLE Subject Specialism Physics common mistakes
GTLE Subject Specialism Physics · Understand the traps and how to avoid them.
When adding or resolving vectors, the candidate sums magnitudes arithmetically (e.g., treats a 3 N and 4 N force at right angles as 7 N) instead of resolving into perpendicular components and combining with Pythagoras or a scale diagram.
School-level habit of adding numbers and a weak grip on the idea that force and velocity are vectors with independent components; candidates also skip drawing a free-body diagram, so the geometry is never made explicit.
What to do instead
Before any calculation, sketch the vectors tip-to-tail and label the angle between them. Practise the routine: choose axes, write Fx = F cosθ and Fy = F sinθ with θ measured from a named axis, sum each direction separately, then recombine. Drill the 3-4-5 and 1-1-√2 cases until component signs are automatic.
On kinematics questions the candidate substitutes into v = u + at or s = ut + ½at² without first fixing a positive direction, so a deceleration or a downward acceleration produces a sign error and a physically impossible answer (e.g., negative time accepted as valid).
Formulae are memorised as symbol patterns rather than as vector relations; the candidate does not treat g or a braking acceleration as carrying a sign relative to the chosen axis.
What to do instead
Adopt a written convention each time: state 'up is positive' or 'direction of motion is positive' at the top of the working. List the five quantities (s, u, v, a, t) with signs before choosing an equation, and always sanity-check whether the sign of the answer makes physical sense.
In ray-diagram and lens/mirror questions the candidate draws rays that do not obey the rules (a ray through the centre of a converging lens bent, or a ray parallel to the axis not passing through the focal point), or applies the thin-lens formula with the wrong sign for a virtual image or diverging lens.
The ray rules are learned as pictures to copy rather than as consequences of refraction at each surface; sign conventions for focal length and image distance are memorised inconsistently between textbooks.
What to do instead
Learn three construction rays and state the rule aloud as you draw each. Fix one sign convention (real-is-positive) and write f, u and v with signs before substituting. Check the answer against the diagram: a virtual image must appear on the same side as the object for a lens.
When asked to explain a stationary-wave or resonance phenomenon, the candidate describes 'waves travelling and cancelling' without identifying the two oppositely travelling waves, the boundary condition (node at a fixed end, antinode at an open end), and the resulting harmonic condition L = nλ/2 or L = (2n−1)λ/4.
Superposition is understood only as 'adding amplitudes', and the candidate has not linked the physical boundary to the allowed wavelengths, so the harmonic number is guessed.
What to do instead
For every such question, first name the two waves and their directions, then mark the boundary condition on a sketch, then derive the wavelength condition from the sketch before calculating frequency. Practise pipes open at one end versus both ends as contrasting cases.
In circuit problems the candidate treats current as something that is 'used up' along a series circuit, or adds parallel resistors by simple addition, or ignores internal resistance when computing terminal potential difference.
Everyday language about electricity being consumed, plus rote use of V = IR without the conservation laws (charge conservation at a junction, energy conservation around a loop) that justify the rules.
What to do instead
Apply Kirchhoff's laws explicitly: sum of currents at a junction, sum of emf and pd around a loop. Use 1/R_total = Σ1/R for parallel branches and check that the parallel combination is smaller than the smallest branch. For cells, write ε = I(R + r) and compute terminal pd as ε − Ir.
On thermal questions the candidate applies pV = nRT or the first law using Celsius temperatures, or confuses heat supplied with change in internal energy when work is done by the gas.
Temperature is treated as an everyday quantity rather than as absolute thermodynamic temperature, and the sign convention for work done by versus on the gas is not fixed.
What to do instead
Convert to kelvin as the first line of every gas calculation and note the conversion in the working. State the first law as ΔU = Q − W with W defined as work done by the gas, then decide the sign of W from whether the volume increases or decreases before substituting.
In photoelectric-effect and nuclear-decay questions the candidate mixes up energy, frequency and wavelength (using E = hf with a wavelength in nanometres without converting), or balances a decay equation on mass number only and not on proton/atomic number.
Unit prefixes are dropped under time pressure, and the conservation rules for nuclear equations are applied as a single check rather than two simultaneous constraints.
What to do instead
Write the unit conversion explicitly before substituting (nm to m, eV to J using 1.6 × 10⁻¹⁹). For every nuclear equation, tally mass numbers and atomic numbers in two separate columns and confirm both balance before moving on.
When reporting a measured quantity the candidate gives an answer with more significant figures than the instrument justifies, omits the unit or the absolute uncertainty, or computes a percentage uncertainty from a mean without using the resolution or the range of readings.
Calculators supply many digits and the candidate equates precision with correctness; uncertainty is treated as an afterthought rather than as part of the measurement.
What to do instead
Adopt a reporting routine: value ± absolute uncertainty with matching decimal places, and a sensible number of significant figures (usually the same as the least precise input). Derive uncertainty from instrument resolution or half the range of repeats, and propagate it through the calculation rather than inventing it at the end.
In practical-skill questions the candidate describes a method without identifying the independent, dependent and control variables, or proposes a safety measure that is generic ('be careful') rather than hazard-specific (e.g., a heat-proof mat under a hot beaker, a sand tray for a falling mass).
The candidate has memorised apparatus lists but not the logic of experimental design, and safety is recalled as a phrase rather than as a risk assessment tied to the specific hazard.
What to do instead
Structure every method answer as: variables (independent, dependent, controls with how each is held constant), apparatus with a reason for each item, a measurement schedule with repeats, and hazards matched to controls. Name the hazard and the specific control in the same sentence.
When planning or marking assessment, the candidate writes recall-only questions ('state the unit of force') for a topic whose learning objective is application or analysis, or sets a mark scheme that awards full credit for a final numerical answer with no allowance for method marks.
The candidate equates coverage of content with coverage of cognitive demand and has not mapped each question to a specific objective or command word; mark schemes are copied from answer keys rather than built from the skills being tested.
What to do instead
Before writing items, tabulate objective, command word (state, explain, calculate, evaluate) and intended demand, then check each question matches its row. Build mark schemes with explicit method marks, unit marks and a stated consequence for a carried-forward error, and moderate against a second script.