LC Physics common mistakes
LC Physics · Understand the traps and how to avoid them.
In a projectile or circular-motion question, draws a separate arrow labelled “centripetal force” alongside tension, weight or normal reaction, then tries to balance it with another force.
The phrase “centripetal force” sounds like an extra force, but it is the resultant of the real forces acting inward.
What to do instead
Draw only real forces on the free-body diagram; choose inward as positive and write the resultant inward force = mv²/r or mω²r; never add centripetal force as an extra arrow.
In a collision or rebound question, drops the sign of velocity after a bounce or treats kinetic energy as a vector when checking whether a collision is elastic.
Candidates copy speeds without assigning direction, and “conservation” is recalled as a slogan rather than a vector statement.
What to do instead
Define a positive direction before substituting; give velocities signs; conserve momentum as a vector sum; calculate kinetic energy as the scalar ½mv² and equate total kinetic energy only for elastic collisions.
When applying Snell’s law, measures the angle between the ray and the glass/water surface instead of the angle between the ray and the normal.
The angle on the diagram is visually convenient and the protractor is often placed along the surface.
What to do instead
Draw the normal at the point of incidence; mark i and r between the ray and the normal; then use n₁ sin i = n₂ sin r, checking that each sine value is less than 1.
In a diffraction-grating calculation, substitutes d = 1/N with N in lines per mm and λ in nm without converting either to metres.
The formula is memorised without unit discipline, and mixed prefixes are common in grating questions.
What to do instead
Convert N to lines per metre so d = 1/N metres; convert λ to metres; solve nλ = d sin θ; then check sin θ ≤ 1 and report θ in degrees.
Calculates the terminal voltage of a cell as V = IR_external and then ignores internal resistance when finding the current or the power delivered to the external circuit.
Emf and terminal potential difference are treated as interchangeable, and internal resistance is invisible in the circuit diagram.
What to do instead
Write E = I(R + r), so terminal p.d. V = E − Ir = IR; use the external R for output power and E I for total power; always include r when calculating current.
In an AC power question, substitutes the peak voltage into P = V²/R to find the average power dissipated in a resistor.
The peak value is the number read from the graph, while rms values are under-practised.
What to do instead
Convert peak to rms first: V_rms = V_peak/√2 and I_rms = I_peak/√2; use P_avg = V_rms²/R or I_rms²R; use peak values only if instantaneous power is explicitly asked for.
In photoelectric-effect questions, claims emission occurs at any frequency if the intensity is high enough, or equates the stopping potential directly to the photon energy.
Classical wave intuition suggests brighter light always gives faster electrons, and hf and eV_s are learned as separate formulas.
What to do instead
Use the photon model: hf = W + E_k,max, with emission only if f > f₀ = W/h; stopping potential gives E_k,max = eV_s, so eV_s = hf − W; intensity affects current, not maximum kinetic energy.
In half-life questions, calculates the fraction remaining after three half-lives as 100% − 3 × 50% or as 100%/3 instead of using repeated halving.
Half-life is misread as a fixed amount removed per interval rather than the time for half of the remaining nuclei to decay.
What to do instead
Use fraction remaining = (1/2)^n where n = t/T½; apply N = N₀(1/2)^n and A = A₀(1/2)^n; after three half-lives, 1/8 remains, not zero and not one-third.
In a heating-curve calculation, uses Q = mcΔT for the melting or boiling section and allows the temperature to change during the phase change.
All heat formulas look interchangeable, and latent heat is not firmly linked to a constant-temperature phase change.
What to do instead
Split the heating curve into stages; use Q = mcΔT only while temperature changes and Q = mL at melting or boiling; keep temperature constant during the phase change and add the energy for each stage.
In gas-law calculations, substitutes Celsius temperatures directly into pV = nRT or into the combined gas law ratios.
Everyday temperature values are in Celsius, and the kelvin conversion is easily forgotten under exam pressure.
What to do instead
Convert to kelvin before any gas-law calculation using T/K = θ/°C + 273.15; use kelvin in pV = nRT and in ratio equations; check that pressure is in pascals and volume in cubic metres.