CSEC Physics common mistakes
CSEC Physics · Understand the traps and how to avoid them.
When calculating average speed or acceleration, candidates add or subtract distances and times in the wrong combination, for example dividing total distance by the sum of the two separate speeds rather than by total time, or using (u+v)/2 as though it applied to non-uniform motion.
The formula triangle for v=d/t is memorised as a single operation, so when a journey is split into two legs, candidates substitute the two speeds into the d/t slot directly. They treat average speed as an arithmetic mean of speeds, which only holds when equal times are spent at each speed.
What to do instead
Before substituting numbers, write a table with columns for distance, time and speed for each leg, then compute total distance and total time separately and divide once at the end. Test the result against a limiting case: if one leg takes almost all the time, the average speed must be close to that leg's speed. Practise a two-leg problem where equal distances are covered at different speeds and show the answer is the harmonic mean, not the arithmetic mean.
On free-body and vector problems, candidates omit one force (usually weight, normal reaction or friction) from the diagram, or resolve a force using the wrong trigonometric ratio, putting the adjacent component as sin θ.
Forces are drawn from memory of similar textbook pictures rather than from a systematic inventory. The sine/cosine swap arises because the angle is marked in different places in different diagrams, and candidates memorise 'sin for vertical' instead of identifying which side is adjacent to the stated angle.
What to do instead
Adopt a fixed checklist for every free-body diagram: weight, contact/normal, tension, friction, applied, and any field force. Label each with its source and draw it from the body's centre. For resolution, always mark the angle between the force and the horizontal (or the stated reference), then write the component along the reference as F cos θ and perpendicular as F sin θ, checking that θ=0 gives the full component along the reference. Verify equilibrium by summing components in both directions.
Candidates confuse mass and weight in calculations, quoting weight in kilograms or using W=mg with g omitted, and they treat the principle of moments as 'forces balance' rather than 'moments balance', ignoring the perpendicular distance.
Everyday language uses 'weight' for mass, and the newton is encountered less often than the kilogram. In moments questions the distance is often given along a slanted rod, and candidates use that length rather than the perpendicular distance from the pivot.
What to do instead
Write units on every quantity as you substitute, and treat a kilogram answer to a force question as an automatic error signal. For moments, redraw the diagram with the perpendicular distance marked by a right-angle symbol from the pivot to the line of action of each force before calculating. Check the direction of each moment (clockwise or anticlockwise) and equate the two sums explicitly.
In heat calculations, candidates mix up specific heat capacity and specific latent heat, applying mcΔT across a phase change or using mL during a temperature rise, and they ignore heat lost to the surroundings when the question implies an insulated system.
Both formulas look similar and both contain 'specific', so the choice is made by pattern-matching the numbers given rather than by identifying what is happening to the substance. Latent-heat steps are also the ones most often skipped in multi-stage heating curves.
What to do instead
Sketch the heating curve first and mark each stage: sloping sections use mcΔT, flat sections use mL. Then write a separate line of working for each stage and sum them. When two bodies are mixed, write 'heat lost by hot = heat gained by cold' as an equation before substituting, and state the assumption of no heat loss explicitly so the examiner sees you have considered it.
Candidates state the direction of heat transfer incorrectly, claiming 'cold flows into the object' or that a metal feels cold because it is at a lower temperature than the hand when both are at room temperature, and they misdescribe conduction, convection and radiation mechanisms.
Sensory experience suggests cold is a substance that enters the hand. The microscopic picture of conduction (lattice vibration and free-electron transfer) and convection (density change causing bulk fluid movement) is learned as a phrase without a physical model, so it is reproduced inaccurately under pressure.
What to do instead
State the rule as 'net heat flows from higher to lower temperature until thermal equilibrium'. For the metal-at-room-temperature case, explain that metal conducts heat away from the hand faster than wood, so the skin cools faster, and that both objects are at the same temperature. For each mechanism, write one sentence naming the carrier of energy: vibrating particles and free electrons for conduction, moving fluid of changed density for convection, electromagnetic waves needing no medium for radiation.
In ray diagrams and lens or mirror calculations, candidates place the image on the wrong side, use the wrong sign convention, or treat a virtual image as real, and in wave questions they confuse frequency with speed when a wave crosses into a new medium.
Sign conventions differ between syllabuses and textbooks, and candidates half-remember one. The wave error comes from thinking that a change of speed changes the frequency, because the two quantities appear together in v=fλ and the roles are not separated.
What to do instead
Write the convention at the top of your answer (for example, real-is-positive with distances measured from the lens) and apply it consistently. Always draw the diagram to scale-ish with at least two standard rays before calculating, and use the diagram to check the sign of your answer. For refraction, state that frequency is set by the source and is unchanged, speed changes with the medium, and wavelength changes in proportion to speed.
Candidates draw or interpret transverse and longitudinal wave representations without labelling wavelength and amplitude correctly, and they calculate wave speed using the distance between two adjacent crests as though it were the time period, or misread a displacement-time graph as displacement-distance.
The same sinusoidal shape is used for both displacement-time and displacement-distance graphs, and the horizontal axis label is skimmed. Wavelength and period are then read off whichever axis is present without checking its units.
What to do instead
Before reading any value, state aloud what the horizontal axis measures and its unit: seconds means period and frequency, metres means wavelength and speed. Mark one full cycle on the graph with a bracket and label it. Then use v=fλ with f=1/T, substituting only after the axis check. For longitudinal waves, describe compressions and rarefactions and note that displacement is along the direction of travel.
When solving series and parallel circuits, candidates add resistances in the wrong rule, assume current is the same everywhere in a parallel network, or apply V=IR to a whole circuit while ignoring internal resistance or the meter's own resistance.
The two rules are learned as a pair and recalled under time pressure without checking the circuit topology. Ideal-meter assumptions are taught but not flagged, so candidates do not notice when a question deliberately gives a non-ideal ammeter or voltmeter.
What to do instead
Redraw the circuit and colour-code each branch before calculating. Write the rule alongside the topology: same current in series, same voltage across parallel branches. Compute the equivalent resistance step by step from the innermost parallel group outwards. Where internal resistance or meter resistance is given, include it as an extra resistor in the model and state that assumption in your answer.
Candidates confuse the direction of conventional current with electron flow, misapply Fleming's left-hand rule to generators or the right-hand rule to motors, and describe electromagnetic induction without mentioning the change in flux linkage.
The two rules share finger and thumb assignments and are easy to swap. Induction is often reduced to 'a wire in a magnetic field gives a current', omitting the requirement for relative motion or a changing flux, so a stationary wire in a steady field is wrongly said to produce an EMF.
What to do instead
Anchor each rule to a device: left hand for a motor (force on a current-carrying conductor), right hand for a generator (induced current). Write 'flux linkage change' as the cause in every induction answer, and check that either the field, the area or the orientation is changing. For current direction, state explicitly that conventional current is from positive to negative and is opposite to electron drift.
In atomic and matter questions, candidates write nuclear equations that do not balance nucleon and proton numbers, confuse alpha and beta decay products, and describe the plum-pudding or Rutherford models with the observations and conclusions swapped.
Alpha and beta particles are memorised as 'helium' and 'electron' without the accompanying change in atomic number, so the daughter nuclide is written incorrectly. The historical experiments are learned as a story rather than as evidence-conclusion pairs.
What to do instead
For every decay equation, write the mass number and atomic number above each symbol and check both sums balance before moving on: alpha lowers A by 4 and Z by 2, beta-minus leaves A unchanged and raises Z by 1. For the models, make a two-column table of observation and conclusion, for example 'some alpha particles deflected through large angles, therefore mass concentrated in a small positive nucleus'.