Pancyprian Technical School Physics (4-hour) common mistakes
Pancyprian Technical School Physics (4-hour) · Understand the traps and how to avoid them.
When converting units in kinematics problems, the candidate mixes minutes with seconds or grams with kilograms and then substitutes the raw numbers straight into s = ut + ½at², producing answers off by factors of 60, 3600 or 1000.
The formula is memorised as a string of symbols, so the candidate focuses on finding a number for each symbol and treats unit consistency as an afterthought. In the pressure of a four-hour paper, converting every quantity feels like wasted time, and mixed units often hide inside word problems (speed in km/h, time in minutes, mass in g).
What to do instead
Before substituting, write a one-line 'given' list with every quantity converted to SI base units and the conversion shown (e.g. 72 km/h = 20 m/s; 250 g = 0.25 kg). Then check the dimensions of the final expression: if the answer's unit does not come out as m, m/s or m/s², the substitution is wrong. Practise ten mixed-unit kinematics items until the conversion step is automatic.
The candidate draws a free-body diagram that omits a force (usually the normal reaction on an incline, tension in a connected-body system, or the component of weight along the slope) and then applies F = ma to the incomplete set, getting a wrong acceleration that looks numerically plausible.
Forces are recalled as a list associated with a situation rather than derived from the object's interactions. Inclined planes and pulley systems add geometric decomposition, and candidates skip the diagram to save time, so the missing force is never noticed.
What to do instead
Adopt a fixed procedure: isolate one body, draw it as a point, mark every contact and long-range interaction, decompose along and perpendicular to the acceleration direction, then write ΣF∥ = ma and ΣF⊥ = 0 separately. Check the perpendicular equation gives the normal reaction before using friction. Do at least five incline-plus-pulley problems using this template.
In work–energy and momentum questions the candidate equates kinetic energy before and after a collision, or applies conservation of momentum to a system where an external impulse acts, without checking whether the collision is elastic or whether the system is isolated.
Both conservation laws are learned as 'energy is conserved' and 'momentum is conserved' slogans, and candidates do not distinguish the conditions under which each applies. Elastic and inelastic collisions look similar in a diagram, and the word 'collision' triggers energy conservation reflexively.
What to do instead
Before writing any equation, state the classification explicitly: 'isolated system → momentum conserved' and 'elastic → kinetic energy also conserved; inelastic → use momentum only, then compute energy loss'. For explosions and recoil, note that kinetic energy increases. Verify by computing total KE before and after for every collision you solve.
For circular motion the candidate uses v = ωr with r taken as the diameter, or forgets that centripetal force is the resultant of the real forces and instead adds an extra outward 'centrifugal force' to the diagram, leading to wrong tensions, normal reactions and minimum speeds.
The term 'centripetal force' sounds like an additional force rather than a role played by tension, weight, friction or gravity. The everyday feeling of being pushed outward in a turning vehicle reinforces the fictitious-force picture, and radius versus diameter confusion is a silent arithmetic slip.
What to do instead
Write the radial equation as ΣF_toward centre = mv²/r and list which real forces supply it (e.g. at the top of a loop: weight + normal = mv²/r). Never draw an outward force on the free-body diagram. Always mark the centre and label r on the diagram before substituting, and check the minimum-speed condition by setting the normal reaction to zero.
In simple harmonic motion the candidate substitutes into x = A sin(ωt) or v = ωA cos(ωt) without checking whether the motion started from the equilibrium position or from maximum displacement, and confuses angular frequency ω with frequency f, producing a phase or factor-of-2π error.
Sine and cosine forms are memorised interchangeably, and the initial condition is treated as irrelevant detail. The relation ω = 2πf is known but not applied consistently, especially when the period is given in milliseconds or when the question asks for a time rather than a displacement.
What to do instead
Always state the initial condition first ('at t = 0, x = 0 moving positive → use sine'). Write ω = 2π/T explicitly and keep the symbol until the final substitution. When asked for a time, solve for the phase angle and convert back with t = θ/ω, checking the answer lies within one period.
In gas-law and thermal problems the candidate substitutes Celsius temperatures into pV = nRT or into p₁V₁/T₁ = p₂V₂/T₂, and treats heat capacity and specific latent heat as interchangeable, adding Q = mcΔT and Q = mL for the same stage of a process.
Everyday temperature reporting is in Celsius, so the numbers on the page feel natural to insert. The distinction between temperature change and phase change is blurred because both appear as 'heating' in a word problem, and the plateau on a heating curve is not connected to the equations.
What to do instead
Convert to kelvin as the first line of every gas calculation and write T = θ + 273. For thermal-energy problems, sketch the temperature–energy graph and label each segment as 'warming' (mcΔT) or 'melting/boiling' (mL) before writing any equation. Check that no segment uses both formulas.
When solving direct-current circuits the candidate adds parallel resistors as if in series, or applies the terminal-voltage relation incorrectly by ignoring internal resistance, so the current through a branch and the voltmeter reading across the battery disagree with Kirchhoff's laws.
The reciprocal formula for parallel resistors is memorised as a rule without the underlying idea that parallel branches share the same voltage. Internal resistance is invisible in the circuit diagram, so it is dropped, and candidates do not check their answer against energy or charge conservation.
What to do instead
Redraw the circuit marking nodes and label which components share a node (same voltage). Compute equivalent resistance step by step, series first then parallel, and write the reciprocal formula out. Always include r in the emf equation ε = I(R + r) and check that the terminal voltage is less than ε when current flows. Verify branch currents sum at each junction.
In electromagnetic induction the candidate applies ε = Bℓv or ε = −N ΔΦ/Δt without checking that the conductor, field and velocity are mutually perpendicular, and states the direction of the induced current by guessing rather than applying Lenz's law to the change in flux.
The formula is remembered as a product of three symbols, and the vector geometry that makes the product valid is not visualised. Lenz's law requires reasoning about whether flux is increasing or decreasing, which is a second step candidates skip under time pressure.
What to do instead
Before substituting, sketch the field lines, the conductor and the velocity, and mark the angle between them; if not perpendicular, use the component B sin θ or ℓ sin θ. For direction, write two sentences: 'flux into the page is increasing, so the induced current must produce flux out of the page, hence by the right-hand grip rule the current is anticlockwise.' Practise this wording on five coil problems.
In geometric optics and sound problems the candidate uses the thin-lens or mirror equation with distances measured from the wrong reference point or with a sign convention applied inconsistently, and treats the Doppler shift or the speed of sound as independent of the medium, giving an image position or frequency that contradicts a ray diagram.
Sign conventions differ between textbooks and are memorised as a table rather than derived from a ray diagram. For sound, the everyday experience that pitch changes with motion is not linked to the formula, and the role of the medium in setting wave speed is overlooked.
What to do instead
Draw the ray diagram first, using two principal rays, and read off whether the image is real or virtual and on which side. Then assign signs according to one stated convention (real-is-positive) and keep it for the whole paper. For sound, identify source velocity, observer velocity and medium velocity separately and state the medium before computing the wave speed.
In the laboratory section the candidate records raw measurements without estimating uncertainty, quotes a final result to more significant figures than the instrument allows, and plots a graph without labelling axes, units or error bars, then reads a gradient from a single pair of points rather than a line of best fit.
School practical work often rewards getting the 'right' number, so uncertainty analysis feels like decoration. Significant figures are copied from the calculator display, and the gradient is computed from the first and last data points because that is quicker than drawing a best-fit line.
What to do instead
For every measured quantity record the instrument resolution and quote the value as (reading ± resolution). Carry uncertainties through by the fractional method for products and quotients. Round the final answer to the same number of significant figures as the least precise input. On graphs, label both axes with quantity and unit, draw a best-fit line, and take the gradient from two widely separated points on that line, not from data points.