ORT Physics common mistakes
ORT Physics · Understand the traps and how to avoid them.
Using the SUVAT equations (e.g. s = ut + ½at²) for a motion where acceleration is not constant, such as a mass oscillating on a spring or a body moving with velocity-dependent drag.
Textbook problems drill the constant-acceleration formulas so heavily that they become the default reflex; candidates recognise a kinematics scenario and reach for the familiar equation without first checking whether a is uniform.
What to do instead
Before selecting any equation, write down explicitly whether acceleration is constant and why (e.g. 'a = g, constant, free fall'). If a varies, either split the motion into intervals with constant a, or integrate a(t) / use energy methods. Do one practice problem each with SHM, drag, and a varying applied force, and note the equation you were tempted to use versus the valid one.
Treating a force as a property of a body rather than of an interaction, so free-body diagrams omit the reaction partner or show 'the centripetal force' as an extra arrow alongside weight and tension.
Everyday language ('the force of the object') and formula sheets that list Fc = mv²/r as if it were a separate force encourage double counting; candidates add the centripetal force on top of the real forces instead of identifying which real force supplies it.
What to do instead
Draw the free-body diagram first, labelling only forces exerted by identifiable agents (gravity, normal, tension, friction). Then write ΣF = ma and let the radial component of the net force equal mv²/r. Never draw a separate 'centripetal' or 'centrifugal' arrow. Practise the conical pendulum and banked-curve cases until the habit is automatic.
Applying pV = nRT or the first law with temperatures in degrees Celsius, or using the wrong sign convention for work done on versus by the gas.
Laboratory data are often recorded in °C and copied straight into the gas equation; sign conventions for W differ between textbooks, so the candidate mixes a convention learned in one chapter with a formula from another.
What to do instead
Convert to kelvin in the first line of every thermodynamics solution (T/K = θ/°C + 273.15). State your convention explicitly at the top of the answer ('W = work done by the gas'), and keep it consistent through ΔU = Q − W. Check by testing an isothermal expansion: Q should be positive and W positive.
Assuming all molecules in a gas move at the mean speed, and using ½mc² with c = rms speed interchangeably with mean speed when computing kinetic energy.
The kinetic-theory result ½m⟨c²⟩ = (3/2)kT is often memorised without the distinction between rms, mean and most-probable speeds, and the Maxwell–Boltzmann distribution is treated as a single value rather than a spread.
What to do instead
Learn the three speeds and their ratios (rms : mean : most probable ≈ 1.73 : 1.60 : 1.41 for a given gas). When a question asks for kinetic energy, use the mean square speed (rms) and say so; when it asks for collision rate or mean free path, use the mean speed. Sketch the distribution and mark all three to fix the distinction visually.
Applying Kirchhoff's loop rule while ignoring the internal resistance of a cell or the potential drop across an ammeter, then being surprised when the measured terminal voltage is below the emf.
Circuit diagrams in exercises often draw ideal cells and ideal meters, so candidates carry the ideal model into laboratory or multi-loop problems where the real drops matter.
What to do instead
Annotate every real source as an emf in series with an internal resistance r, and every real meter with its resistance. Write terminal voltage as V = ε − Ir and check it against the measured value. In the lab, measure emf by extrapolating a V–I graph to I = 0 rather than reading the open-circuit voltage of a loaded cell.
Using the right-hand rule for a magnetic force but assigning the direction of conventional current to electron flow, or applying F = BIl sinθ to a wire that is not perpendicular to B without the sinθ factor.
The rule is taught with a hand gesture that is easy to mirror-reverse, and current direction (conventional versus electron) is a recurring source of sign errors; the sinθ dependence is dropped when the geometry looks 'roughly perpendicular'.
What to do instead
Standardise one gesture and always label the conventional current arrow on the diagram before applying the rule. For any wire or moving charge, write the angle θ between v (or I) and B explicitly and keep sinθ. Verify with a limiting case: a wire parallel to B must feel zero force.
Using the lens or mirror equation with distances measured from the wrong reference point, or treating a virtual image distance as positive without checking the sign convention.
The Cartesian (real-is-positive) and the 'real-is-positive for object, negative for virtual image' conventions coexist in textbooks, and candidates switch conventions mid-problem, especially with diverging lenses and convex mirrors.
What to do instead
Choose the Cartesian convention once: measure all distances from the pole/optical centre, positive in the direction of incident light. Write the sign of each quantity before substituting. Cross-check with a ray diagram - if the construction gives a virtual image, the image distance must come out negative.
Computing a single-slit or diffraction-grating pattern using the wrong condition, e.g. using d sinθ = nλ for minima of a single slit, or forgetting that a grating's maxima are much sharper and the slit width sets the envelope.
The formulae for single-slit minima (a sinθ = nλ) and grating maxima (d sinθ = nλ) look identical, and candidates memorise one pattern without attaching it to the physical situation.
What to do instead
Attach each formula to its physical feature: slit width a controls the diffraction envelope (minima), slit separation d controls interference maxima. Label a and d on a sketch before calculating, and note that for a grating d ≫ a. Check the maximum order by setting sinθ = 1.
Balancing a nuclear equation on mass number only, or ignoring the neutrino and the Q-value when deciding whether a beta decay is energetically possible.
Alpha and beta decays are often practised as simple A and Z bookkeeping, and the continuous beta spectrum (requiring an antineutrino) is treated as a footnote rather than a constraint on the equation.
What to do instead
For every decay write both A and Z conservation explicitly, and include the neutrino/antineutrino for beta processes. Then compute the mass defect of the products versus the parent and confirm Q > 0. Practise one example each of α, β⁻, β⁺ and electron capture, checking charge and lepton number.
Quoting a final result to more significant figures than the data allow, or combining uncertainties by simple addition when the quantities are multiplied or raised to a power.
Calculators return many digits and candidates copy them across; percentage and fractional uncertainties are taught separately from absolute ones, so the rule for products and powers is not applied at the point of calculation.
What to do instead
Carry full precision through the working, then round the final answer to the least number of significant figures in the input data. For products and quotients add fractional uncertainties; for a power n multiply the fractional uncertainty by n. Write the uncertainty on the same line as the value, e.g. (3.2 ± 0.1) m s⁻¹.