BGCSE Physics 1427 common mistakes
BGCSE Physics 1427 · Understand the traps and how to avoid them.
In moments questions, multiplying the force by the whole length of a lever or beam instead of by the perpendicular distance from the pivot to the line of action of the force.
Diagrams label a length that is not perpendicular to the force, and candidates grab the most visible number rather than re-drawing the geometry.
What to do instead
Extend the force line, drop a perpendicular from the pivot, label that distance d, and only then write moment = F × d; check that d is shorter than or equal to the actual lever length.
Treating the weight of an object and the normal force on it as a Newton's third-law pair because they are equal and opposite in a stationary case.
The rule 'equal and opposite' is remembered without the requirement that the two forces act on different bodies.
What to do instead
For every third-law pair, name the two bodies involved: 'Earth pulls book' pairs with 'book pulls Earth', not with 'table pushes book'; balanced forces act on the same body, third-law pairs on two bodies.
Calculating the total resistance of parallel resistors by adding the resistance values directly, or writing 1/R = 1/R1 + 1/R2 and forgetting to invert the result.
The series formula is learned first and is more intuitive; the reciprocal step is easily dropped under time pressure.
What to do instead
Always finish parallel calculations with R = 1/(sum of reciprocals), and sanity-check that the total is smaller than the smallest branch resistance.
Connecting an ammeter in parallel with a component or a voltmeter in series with it when converting a circuit diagram into a real circuit.
Both instruments are drawn as circles with letters, and the rule 'ammeter in series, voltmeter in parallel' is recalled but not applied to the drawing.
What to do instead
Trace the current path before connecting: break the circuit to insert the ammeter so the same current must pass through it; attach the voltmeter's two leads to the two ends of the component only.
Measuring the angle of incidence or refraction between the ray and the mirror or water surface rather than between the ray and the normal.
Protractors are naturally aligned with the surface, and the normal is an invisible construction line that candidates omit.
What to do instead
Draw the normal as a dashed line at 90° to the surface at the point of incidence, mark the angles from that line, and state in the answer 'angle to the normal'.
Substituting frequency in kHz or wavelength in cm directly into v = fλ and reporting a speed that is wrong by a factor of 1000 or 100.
The wave equation is treated as a number-plugging exercise; unit prefixes are ignored because the numbers look convenient.
What to do instead
Convert every quantity to SI before substitution (kHz × 10^3, cm × 10^-2), write the unit conversion as a separate line, and check that the speed is in a realistic order of magnitude for the wave concerned.
Using Q = mcΔT to find the energy needed to melt ice at 0 °C, or applying a temperature change during a change of state.
Specific heat capacity and specific latent heat are both 'energy per kilogram' formulas and are recalled interchangeably.
What to do instead
Ask whether the temperature is changing: if yes use mcΔT, if a state change is occurring at constant temperature use mL; for a full heating curve split the process into separate stages and add the energies.
Stating that after two half-lives no radioactive nuclei remain, or that the half-life is the time for the count rate to reach zero.
Half-life is understood as 'halving' but the exponential, never-zero nature of decay is not modelled.
What to do instead
Use a table: start with 100 %, after 1 half-life 50 %, 2 → 25 %, 3 → 12.5 %; state explicitly that the activity halves each half-life and approaches zero without reaching it.
Recording a stopwatch or metre-rule reading to more decimal places than the instrument can justify, such as 24.35 cm on a millimetre rule, or writing a numerical answer without a unit.
Calculators display many digits and candidates copy them; units are treated as decoration rather than part of the value.
What to do instead
Decide the precision from the smallest scale division, round the final answer to the same or one more significant figure, and write the unit at every step.
Drawing a dot-to-dot line through plotted points instead of a single line of best fit, then calculating the gradient from two data points rather than from the fitted line.
Joining dots feels like 'following the data', and the gradient formula is applied to raw table values.
What to do instead
Draw one smooth straight line with points scattered evenly above and below it, choose two widely spaced points on that line (not data points), and calculate gradient = Δy/Δx from those coordinates.