IB Physics SL common mistakes
IB Physics SL · Understand the traps and how to avoid them.
When combining quantities in a formula, the candidate adds the percentage uncertainties of all quantities, including those that are squared or cubed, instead of multiplying the percentage uncertainty by the exponent before adding.
The candidate memorises 'add percentage uncertainties when multiplying' and applies it mechanically, forgetting that a term like r³ or v² is a repeated multiplication whose uncertainty compounds with the exponent.
What to do instead
Rewrite the formula in terms of only multiplication and division, then apply the rule: total percentage uncertainty is the sum of (percentage uncertainty of each quantity × its exponent), taking absolute values. Practise with T = 2π√(l/g) and KE = ½mv² until the exponent handling is automatic.
The candidate reports a final answer to 5 or 6 significant figures and with an uncertainty quoted to several figures, for example 9.8137 ± 0.2461 m s⁻².
The candidate treats the calculator display as the answer and has not internalised that the uncertainty determines how many figures are meaningful.
What to do instead
Round the absolute uncertainty to one significant figure (two only if the leading digit is 1), then round the value to the same decimal place. Check every numerical answer against this rule before moving on; it is a quick, mechanical mark.
On projectile or inclined-plane problems, the candidate resolves the weight into components but then also uses the full weight in the same direction, or applies sin and cos to the wrong angle.
The candidate draws the free-body diagram hastily and does not mark which angle in the triangle corresponds to the slope angle; the component perpendicular to a slope is mg cos θ, but this is memorised without a diagram.
What to do instead
Always draw the free-body diagram with the weight vector and the slope, then construct the right triangle geometrically so the correct component is derived rather than recalled. Verify with limiting cases: θ = 0 should give zero component along the slope and full mg perpendicular.
The candidate equates the tension in a string to the weight of the hanging mass even when the system is accelerating, or assumes the reading on a scale in a lift always equals mg.
Newton's second law is applied to individual objects inconsistently; the candidate forgets that tension is a constraint force determined by the whole system's acceleration, not by static equilibrium.
What to do instead
Write separate F = ma equations for each body, define the positive direction for the system, and solve simultaneously. For lift problems, explicitly write N − mg = ma with a signed acceleration so the apparent-weight result follows rather than being guessed.
The candidate uses Q = mcΔT and then, in the same problem, applies Q = mL to the same mass without checking whether a phase change occurs, or uses the wrong specific heat for the phase present.
The candidate treats the two formulas as interchangeable 'heat' equations and does not sketch the temperature–energy graph, so the stages of heating, melting, and heating again are not separated.
What to do instead
Before calculating, sketch the temperature against energy supplied and mark each stage. Apply Q = mcΔT only within a single phase and Q = mL only at a constant-temperature transition, then sum the stages. State the phase for each term explicitly.
The candidate assumes a wave's frequency changes when it passes from one medium to another, or changes the frequency when calculating the Doppler effect for a moving source.
The candidate confuses the quantities that are conserved at a boundary with those that change; wavelength and speed change with medium, but frequency is set by the source.
What to do instead
Anchor on the principle that frequency is determined by the source and is unchanged at a boundary; use v = fλ to find the new wavelength. For Doppler, apply the shift to the observed frequency only, and check the sign by asking whether the source is approaching or receding.
The candidate adds resistor values in parallel, or uses the supply EMF as the terminal potential difference across the external circuit without subtracting the lost volts across internal resistance.
The candidate applies series intuition to parallel networks and does not distinguish EMF from terminal p.d. when a current flows through internal resistance.
What to do instead
For parallel branches compute 1/R_total = Σ1/R_i and check the result is smaller than the smallest branch. Always write ε = I(R + r) and V_terminal = ε − Ir, then verify that V_terminal equals IR across the external load.
The candidate states that a satellite in a circular orbit has zero gravitational field strength, or claims that the centripetal force does work on the satellite and changes its speed.
The candidate conflates 'weightlessness' (free fall) with the absence of gravity, and does not connect the centripetal force to the velocity being perpendicular to it.
What to do instead
Emphasise that g = GM/r² is non-zero at orbital altitude and provides the centripetal force: GMm/r² = mv²/r. Note that the force is perpendicular to velocity, so no work is done and speed is constant in a circular orbit.
The candidate writes a nuclear decay equation that balances charge but not nucleon number, or assigns the emitted beta particle the wrong sign for β⁺ versus β⁻ decay.
The candidate balances only the obvious quantity and has not established a fixed procedure for conserving both A and Z across the reaction.
What to do instead
For every nuclear equation, write two conservation checks side by side: sum of A on each side, sum of Z on each side. For β⁻, a neutron converts to a proton and an electron plus antineutrino; for β⁺, a proton converts to a neutron and a positron plus neutrino. Verify both rows before finalising.
The candidate calculates the efficiency of an energy-production system using the energy input and output but mixes units or ignores that Sankey diagrams must conserve total energy including wasted transfers.
The candidate treats efficiency as a formula to plug into without checking unit consistency or accounting for all output pathways shown in the diagram.
What to do instead
Convert all quantities to the same unit (usually joules) before dividing, then compute efficiency = useful output / total input and confirm it is below 100%. When reading a Sankey diagram, check that the widths of all outgoing arrows sum to the input width.