Valencian Community PAU Physics common mistakes
Valencian Community PAU Physics · Understand the traps and how to avoid them.
In orbital-mechanics problems, the candidate equates gravitational force to centripetal force but then substitutes the satellite's altitude for r, writing GMm/h² = mv²/h instead of using the distance from the centre of the planet (r = R + h).
The formula sheet and textbook diagrams often label the orbital radius simply 'r', and the statement gives the altitude above the surface. Under exam pressure the candidate grabs the only number in the text and drops the planetary radius, because the distinction between 'height above the surface' and 'distance to the centre' was never made automatic.
What to do instead
Before substituting, write one line under the data: 'r = R + h = ... m'. Then derive v = sqrt(GM/r) and T = 2π·sqrt(r³/GM) symbolically first, and only substitute numbers afterwards. Practise three or four problems where the altitude is deliberately comparable to the radius so the error becomes visible.
When asked for the gravitational potential energy or potential at a point between two masses, the candidate adds the magnitudes of the two potentials instead of adding them with their signs, forgetting that gravitational potential is always negative and is a scalar.
Students transfer the vector habit from force problems (where the two forces may genuinely oppose and partly cancel) to potential, and also treat 'potential energy' as a positive quantity because the formula is often memorised as GMm/r without the minus sign.
What to do instead
Write explicitly V = -GM1/r1 - GM2/r2 with the minus signs, and state in one sentence that V is a scalar so no vector decomposition is needed. Check the sign of the answer against the physical statement that a mass at infinity has zero potential.
In simple-harmonic-motion problems the candidate uses x = A sin(ωt) with the given initial position substituted directly for the phase, or forgets that ω = 2π/T (not ω = T or ω = f), producing an amplitude or period that does not match the data.
The general equation is memorised as a template, and the phase constant φ is either ignored or assumed to be zero. The angular frequency is confused with the ordinary frequency because both are called 'frequency' in the statement.
What to do instead
Always start from the general form x = A sin(ωt + φ) and determine A and φ from the initial conditions (x(0) and v(0)) with two explicit equations. Write ω = 2π/T = 2πf in the margin before computing anything. Verify that the resulting velocity amplitude is Aω, not A/T.
In wave problems the candidate confuses the speed of the wave with the speed of the oscillating particles, or applies v = λf to a standing wave and claims the wave transports energy along the string.
Both quantities are called 'velocity' and the same symbol v appears in both contexts. The distinction between a travelling wave (which transports energy) and a standing wave (which does not) is stated once in class but not reinforced with a numerical check.
What to do instead
For every wave question, write two separate lines: 'wave speed v = λf (propagation)' and 'particle speed v_max = Aω (oscillation)'. For standing waves, add the sentence 'no net energy transport; nodes have zero amplitude'. Compare the two numbers explicitly so the difference in magnitude becomes intuitive.
In electrostatics the candidate computes the electric field at a point as the scalar sum of the magnitudes of the fields from two charges, ignoring that the field is a vector and that the fields may point in different directions, so the resultant is not the arithmetic sum.
The potential problem (a scalar sum) was practised just before, and the same numerical habit is carried over. The candidate also draws no diagram, so the geometry of the two contributions is never visualised.
What to do instead
Draw the point and both charges, draw each field vector with its correct direction (away from positive, towards negative), then decompose into components and add them vectorially. Contrast this with a potential calculation on the same configuration so the scalar-versus-vector difference is explicit.
In magnetic-force problems the candidate applies F = qvB without the sine of the angle between v and B, or applies F = BIL to a charged particle, and fails to determine the direction of the force using the right-hand rule correctly.
The magnitude formula is memorised in its simplest perpendicular case, and the vector product is treated as a scalar multiplication. The right-hand rule is often learned as a gesture without a consistent convention for positive and negative charges.
What to do instead
Always write F = qv × B in vector form, then |F| = |q|vB sin θ, and state θ explicitly. For direction, use the right-hand rule for v × B and then reverse the result if q is negative. Check the answer by asking whether the force is perpendicular to both v and B.
In geometric optics the candidate uses the thin-lens or mirror equation with the wrong sign convention, for example placing a virtual image on the same side as the real image, or treating a diverging lens as converging because f is given as a positive number.
Different textbooks use different sign conventions, and the candidate mixes them. The statement may give the focal length as a magnitude, and the candidate substitutes it without deciding the sign from the type of lens.
What to do instead
Adopt one convention (for example, real-is-positive for object and image distances, and f negative for diverging lenses) and write it at the top of the answer. Before substituting, state 'converging lens, f = +...' or 'diverging lens, f = -...'. Check the sign of the magnification and whether the image is real or virtual.
In relativity problems the candidate applies the time-dilation and length-contraction formulas with the proper time and proper length swapped, or uses the Galilean velocity addition for speeds close to c.
The labels 'proper time' and 'proper length' are abstract, and the formulas look symmetric. The candidate also relies on everyday intuition that velocities simply add.
What to do instead
Define proper time as the interval measured by the clock present at both events (same place) and proper length as the length measured in the rest frame of the object. Write 'moving clock runs slow: Δt = γΔτ' and 'moving length contracts: L = L0/γ' as two separate labelled lines. For velocity addition, always use u = (u' + v)/(1 + u'v/c²) and check that the result never exceeds c.
In photoelectric-effect and quantum problems the candidate uses the photon energy hf as the maximum kinetic energy of the emitted electron, or forgets the work function, writing E_k = hf instead of E_k = hf - W.
The energy-conservation step is compressed into a single formula in the notes, and the candidate treats the photon energy as if it were entirely transferred to the electron's kinetic energy.
What to do instead
Write the energy balance explicitly: 'photon energy hf = work function W + maximum kinetic energy E_k'. Compute hf and W separately in electron-volts or joules before subtracting. Check that the threshold frequency satisfies hf0 = W and that no emission occurs below it.
In experimental data-analysis questions the candidate reports a result with more significant figures than the data allow, omits the uncertainty, or plots a graph without labelling axes and units, and reads a gradient without specifying its units.
The numerical answer is treated as the goal, and the treatment of uncertainties and graph conventions is seen as cosmetic. The candidate also copies calculator output directly without rounding.
What to do instead
Follow a fixed reporting routine: compute the mean, the absolute uncertainty (from the instrument resolution or the spread), round the uncertainty to one significant figure, then round the result to the same decimal place. On every graph, label both axes with quantity and unit, draw the best-fit line, and state the gradient with its units and its physical meaning.