JCE Physics common mistakes
JCE Physics · Understand the traps and how to avoid them.
Working in centimetres or grams throughout a calculation and reporting the answer without converting to SI base units, then losing marks on the unit or magnitude.
The question stem supplies data in cm, g, or ms because that is convenient for the scenario, and candidates substitute those numbers directly into formulas that assume metres and kilograms, so the arithmetic is right but the answer is off by powers of ten.
What to do instead
Before substituting, write a one-line 'given' list converting every quantity to SI base units (m, kg, s, A, K, mol, cd) and keep the conversion visible in the working. Do a sanity check on magnitude: a 5 cm pendulum should give a period near 0.45 s, not 4.5 s or 0.045 s.
Treating a vector quantity as a scalar: adding speeds instead of velocities, ignoring direction in momentum or impulse problems, or reporting a negative sign as if it were an error.
Most early practice is one-dimensional and sign conventions feel arbitrary, so candidates drop signs to 'simplify' and then cannot reconcile the result with the physical situation.
What to do instead
At the start of every mechanics problem, draw an axis and state which direction is positive in writing. Resolve all vectors into components before combining, and interpret the final sign in words ('the ball moves opposite the chosen positive direction') rather than deleting it.
Applying conservation of energy or conservation of momentum to a situation where the condition fails, for example using energy conservation across an inelastic collision or momentum conservation when an external force acts during the interval.
Both principles look like universal shortcuts, and candidates pattern-match on keywords such as 'collision' or 'slides down' without checking whether the system is isolated or whether mechanical energy is dissipated.
What to do instead
Before choosing a principle, write the system boundary and list external forces or energy transfers across it. If the collision is inelastic, use momentum for the collision and then energy accounting afterwards, including the heat or deformation term. State explicitly why the chosen conservation law holds.
Confusing the direction of a force with the direction of motion, leading to claims that a centripetal force does work, that a reaction force is 'cancelled' by an action force on the same body, or that a stationary object has no forces on it.
Everyday intuition treats force as a push that must point where things go, and Newton's third law pairs are taught as balanced pairs without stressing that they act on different bodies.
What to do instead
Draw a free-body diagram for one chosen object only, with every force labelled by its source. Check each pair: action and reaction must be on two different bodies and never appear in the same diagram. For circular motion, note that the centripetal force is perpendicular to velocity and therefore does zero work.
Using the mirror or thin-lens equation with the wrong sign convention, or assuming a virtual image is the same as a diminished one, then misclassifying the image as real or virtual.
Different textbooks use different sign rules (real-is-positive versus Cartesian), and candidates mix rules from memory under time pressure.
What to do instead
Commit to one convention and write it at the top of the page before starting optics questions: for example, real-is-positive with focal length positive for converging lenses and mirrors, image distance negative for virtual images. Verify the classification with a quick ray sketch rather than trusting the algebra alone.
Confusing resistance, resistivity, and the effect of changing geometry: assuming doubling the length halves the resistance, or that a thicker wire has higher resistance.
The symbols are similar and the formula R = ρL/A is recalled without the proportional reasoning, so candidates invert the length and area relationships.
What to do instead
Derive the dependence once from R = ρL/A and state it in words: resistance rises with length and falls with cross-sectional area. Practise with ratios ('if L doubles and A halves, R becomes four times larger') and check against the physical picture of electrons squeezing through a narrower pipe.
Mixing up series and parallel rules for resistors and capacitors, or applying resistor rules to capacitors, so equivalent values move the wrong way.
The rules are mirror images of each other, and candidates memorise 'add them up' without attaching it to the quantity being added.
What to do instead
Anchor each rule to a physical statement: in series the same current flows through every element, so resistances add; in parallel the same voltage appears across every branch, so conductances add. For capacitors, charge is shared in series and voltages are shared in parallel. Re-derive the equivalent expression from these statements in the exam rather than recalling it.
Assuming a wave's frequency changes when it passes into a new medium, or misapplying v = fλ by changing two variables at once.
The equation is treated as three independent numbers rather than a relation with a constraint, and candidates do not recall that frequency is set by the source and stays constant across a boundary.
What to do instead
State the constraint before calculating: at a boundary, frequency is unchanged because it is determined by the source, so speed and wavelength change together. Use v = fλ once on each side of the boundary with the same f, and check the direction of change against the refractive index.
Reporting an answer with more significant figures than the data support, or omitting units and the direction of an uncertainty, in a practical or data-analysis question.
Calculators return eight digits and candidates copy them, and precision rules are treated as cosmetic rather than part of the mark scheme.
What to do instead
Identify the least precise given value first and round the final answer to that many significant figures, keeping one extra digit in intermediate steps. Always attach units, and for uncertainties give the value with its absolute uncertainty and the same number of decimal places, for example 9.81 ± 0.02 m s⁻².
Describing a graph's shape or reading a gradient or area without connecting it to the physical quantity, for example saying 'the line is straight' instead of 'acceleration is constant'.
Graph questions are read as geometry exercises, and the link between slope or area and the physical relationship is not rehearsed.
What to do instead
For every graph, first write the defining relation (for example v = ds/dt or W = ∫F dx), then state what the gradient and the area represent with units. Answer in terms of the physics: 'the gradient of the displacement–time graph is velocity, and its constant value shows uniform motion'.