Cambridge IAL Physics common mistakes
Cambridge IAL Physics · Understand the traps and how to avoid them.
Writing the answer to a calculation to the number of decimal places shown on the calculator display, e.g. quoting 9.80665 m s⁻² or 0.333333333 for g or for a ratio, and then carrying that string of digits into the final answer line.
Candidates treat the calculator readout as the answer rather than as an intermediate value, and Cambridge mark schemes for IAL Physics award significant-figure marks separately from the value mark, so an over-precise answer can lose a mark even when the arithmetic is right. There is also a widespread belief that 'more digits means more accurate'.
What to do instead
Before every calculation, note the least precise given data and round the final answer to that many significant figures (usually 2 or 3), keeping full precision only in the working line. Practise the habit of writing the unrounded value in the working and a separately boxed, correctly rounded final value, so the two are visibly distinct.
Treating a vector quantity as a scalar in a mechanics problem - for example adding a 3.0 m s⁻¹ velocity and a 4.0 m s⁻¹ velocity as 7.0 m s⁻¹ when they are perpendicular, or quoting a momentum change without direction.
Candidates learn the scalar arithmetic of speed and distance early and transfer it to velocity, momentum and force without switching to a vector picture. Diagrams in the question often show directions, but candidates read the numbers and ignore the geometry.
What to do instead
Adopt a fixed routine: for any quantity that can have direction, draw a labelled arrow sketch first, then resolve into perpendicular components before adding. State magnitude and direction explicitly in the answer line. When two vectors are at right angles, use Pythagoras for magnitude and tan⁻¹ for direction rather than direct addition.
Applying 'F = ma' to a system without first drawing a free-body diagram, so that weight, normal contact force, tension and friction are double-counted, omitted, or applied to the wrong body (for example including the weight of a hanging mass in the equation for the trolley alone).
Newton's second law is stated for a specified body, but candidates apply it to 'the situation' as a whole. They also confuse the force a body exerts with the force exerted on it, which matters for Newton's third law pairs.
What to do instead
Draw a separate free-body diagram for each body, with only the forces acting on that body, and mark the positive direction. Write ΣF = ma along that direction. For connected bodies, either treat them as one system (internal tensions cancel) or write one equation per body and solve simultaneously - never mix the two approaches in one line.
Describing wave behaviour by naming the phenomenon instead of explaining it in terms of path or phase difference - e.g. answering 'because of interference' or 'because of superposition' when asked why a point in a two-source pattern is a minimum.
Textbook headings and mark-scheme command words are easily memorised as labels, and candidates assume the label is the explanation. IAL mark schemes usually require the physical mechanism (path difference = (n + ½)λ, or phase difference = π, 3π, …) and reward it with the mark.
What to do instead
For any superposition question, always write the quantitative condition: state the path difference or phase difference, compare it with λ or 2π, and only then name the effect. Practise by converting every 'explain the pattern' question into a sentence containing a number and λ.
Mixing up the direction of electromagnetic induction reasoning - claiming the induced current 'opposes the field' rather than opposing the change in flux, or predicting the wrong direction of induced current when a magnet approaches or recedes from a coil.
Lenz's law is often memorised as a slogan without a working procedure. The subtlety is that the induced field opposes the change in flux linkage, so the induced field direction reverses when the magnet recedes even though the magnet's own field has not reversed.
What to do instead
Use a fixed three-step procedure: (1) identify the direction of the external field through the coil, (2) decide whether flux linkage is increasing or decreasing, (3) choose the induced field direction that opposes that change, then apply the right-hand grip rule to get the current direction. Work through one approach and one recession case side by side to see the reversal.
Treating the photoelectric equation as energy conservation with the wrong terms - writing hf = φ + ½mv² but substituting the threshold frequency for f, or using the work function as the maximum kinetic energy, or omitting that current saturation depends on intensity while stopping potential depends on frequency.
The equation is short and easy to memorise symbolically, but the physical meanings of φ, f₀ and the saturation current are separate ideas that candidates merge into one. Graphs of current against voltage then get misread.
What to do instead
Anchor each symbol to a measurement: φ = hf₀ is found from the intercept on a frequency axis of maximum kinetic energy; the stopping potential gives maximum kinetic energy directly; saturation current scales with intensity at fixed frequency. Redraw the current–voltage graph for two intensities and two frequencies and label which feature changes in each case.
Using a formula from the data booklet without checking that the symbols match the physical situation - for example using the ideal-gas equation with Celsius temperatures, or substituting a diameter where a radius is required in a resistance or stress calculation.
The booklet supplies formulas stripped of context, so candidates pattern-match the letters in the question to the letters in the formula and skip the unit and definition check. Area and radius confusion is especially common in Young modulus and resistivity questions.
What to do instead
Before substitution, write the units beside every symbol and convert to SI (K, m, kg, s, A). For any formula containing r or d, write explicitly 'r = d/2 = …' as a line of working. Treat the data booklet as a reference to be checked, not a lookup table to be copied.
Answering a 'describe an experiment to determine X' question with a list of apparatus and no procedure - no statement of what is varied, what is measured, how many readings are taken, or how the graph is used to obtain the quantity.
Candidates recall the apparatus from a textbook diagram but not the logic of the measurement, and they underestimate how much of the mark scheme is allocated to method, control of variables and analysis rather than equipment names.
What to do instead
Structure every experimental description in four labelled parts: measurements (quantity, instrument, resolution), procedure (what is varied, what is held constant, range and number of readings), analysis (which graph, which gradient or intercept gives the answer, with the equation rearranged to match y = mx + c), and safety/precaution. Rehearse this template until it is automatic.
Recording raw readings with too few decimal places or inconsistent precision - writing a stopwatch time as 12 s, a vernier reading as 2.5 cm, or a set of repeated times as 1.2 s, 1.25 s, 1.3 s - then averaging them without comment.
Candidates confuse significant figures in a final answer with the decimal places an instrument can actually deliver, and they have not internalised the resolution of common IAL instruments (metre rule 1 mm, vernier 0.1 mm, micrometer 0.01 mm, stopwatch 0.01 s, digital balance 0.01 g or as marked).
What to do instead
Memorise the resolution of each instrument and write every raw reading to that decimal place, including trailing zeros. For repeated readings, quote the mean to the same decimal place as the raw data and state the uncertainty as half the range or the instrument resolution, whichever is appropriate.
Plotting a graph with a forced line through the origin, joining point to point, or drawing a 'best fit' that ignores the scatter, and then reading a gradient from two data points rather than from the line.
Candidates want the graph to confirm the expected relationship, so they bend the line to pass through the origin even when there is a genuine systematic offset. Point-to-point joining comes from plotting practice in mathematics rather than experimental physics conventions.
What to do instead
Plot points with error bars where uncertainties are given, draw a single straight line of best fit with points scattered evenly either side, and only force the origin if the theory demands it. Take the gradient from two widely separated points on the line (not data points), and read an intercept directly from the axis. State the coordinates used in the working.
Quoting an uncertainty or percentage uncertainty without a basis - writing '± 0.1' with no units, giving a percentage uncertainty larger than 100 %, or adding percentage uncertainties for quantities that are multiplied rather than added.
Percentage uncertainty rules are taught as procedures without the underlying idea that independent fractional uncertainties combine in quadrature or add for products, and candidates do not check whether their result is physically sensible.
What to do instead
Always attach units to absolute uncertainties and express percentage uncertainty as (uncertainty ÷ value) × 100. For a product or quotient, add the percentage uncertainties of the measured quantities; for a sum or difference, add the absolute uncertainties. Sanity-check that no percentage uncertainty exceeds 100 % and that the final quoted uncertainty has one significant figure.