Physics common mistakes
Physics · Understand the traps and how to avoid them.
Using the given value of one quantity as the answer for a different quantity that happens to share a symbol or formula shape, e.g., reporting a particle's speed when the question asks for its acceleration, or reporting the magnitude of a force component when the question asks for the net force.
Under time pressure candidates pattern-match the problem to a memorized formula and then stop at the first number they compute, without re-reading the final question stem. Symbols like v, a, F, and P recur across topics, so the computed quantity feels 'close enough' to the asked quantity.
What to do instead
Before computing, underline the exact quantity requested and its required units, then write a one-line 'target =' statement. After computing, check dimensional units of your result against the target; if the units do not match the requested quantity, you have solved the wrong problem.
Treating Newton's third-law pairs as if they cancel on a single free-body diagram, or omitting the normal force, tension, and static friction from a diagram when they are not 'obvious' forces.
Candidates learn 'action-reaction forces are equal and opposite' and then apply both forces to the same object, producing a false equilibrium. Students also conflate the normal force with weight in incline and elevator problems.
What to do instead
Draw one free-body diagram per object and label only forces acting on that object by other objects. Never include a force the object exerts on something else. For inclines, resolve weight into components and set N = mg cos θ only when there is no other force perpendicular to the surface.
Assuming mechanical energy is conserved in problems involving friction, air resistance, or inelastic collisions, and applying mgh = ½mv² without accounting for non-conservative work.
Energy conservation is taught first in idealized form, and candidates overgeneralize it. Inelastic collisions especially invite the error of setting kinetic energy before equal to kinetic energy after.
What to do instead
State explicitly whether non-conservative forces do work. If yes, use W_nc = ΔE. In collisions, conserve momentum always, but conserve kinetic energy only when the problem states the collision is elastic. Check whether the final kinetic energy is less than or equal to the initial value.
Using the wrong sign convention or forgetting that acceleration due to gravity is negative in projectile motion while horizontal velocity is constant, leading to incorrect time of flight or range.
Candidates mix coordinate conventions mid-problem, or apply a kinematic equation with a nonzero horizontal acceleration. The independence of horizontal and vertical motion is stated but not internalized.
What to do instead
Declare a coordinate system at the start (e.g., up positive). Write the horizontal and vertical equations as separate columns. Use a_y = −g and a_x = 0 unless the problem says otherwise. Solve for time from the vertical equation first, then substitute into the horizontal equation.
In circuit problems, applying the parallel-resistance formula to series resistors (or vice versa), and forgetting that current is the same through series elements while voltage is the same across parallel elements.
The two formulas look symmetric and are memorized as procedures rather than derived from Kirchhoff's laws. Candidates also forget to reduce a mixed network in stages.
What to do instead
Before computing, mark each junction and trace a single path from the source: elements on the same path are in series; elements sharing two nodes are in parallel. Reduce the network step by step, outermost combination first. Verify with Kirchhoff's junction rule that currents sum correctly at each node.
Using the right-hand rule inconsistently for magnetic force and induced current, or forgetting that the magnetic force on a moving charge is perpendicular to both v and B and therefore does no work.
The right-hand rule requires three mutually perpendicular directions, which is hard to visualize without a consistent hand orientation. Candidates also confuse the direction of conventional current with electron flow.
What to do instead
Adopt one fixed hand convention and practice it: thumb = v (or current), fingers = B, palm = F. For induction, use Lenz's law to determine the direction that opposes the change in flux, then check with the right-hand rule. Remember F = qv × B is always perpendicular to v, so magnetic force changes direction but not speed.
Mixing up the sign conventions for image distance, focal length, and magnification in thin-lens and mirror problems, and reporting a virtual image as real (or upright as inverted).
Conventions differ between textbooks and between lenses and mirrors, and candidates memorize a single sign rule that does not transfer. The distinction between real and virtual images is often glossed over.
What to do instead
Write the sign convention at the top of every optics problem: for a converging lens, f is positive; for a diverging lens, f is negative; a negative image distance means a virtual image on the same side as the object. Use 1/f = 1/do + 1/di and m = −di/do, then interpret the sign of m as inverted (negative) or upright (positive).
Applying the ideal-gas law with temperature in degrees Celsius, or using the wrong gas constant for the units of pressure and volume given.
Candidates routinely plug in 25 instead of 298, and they treat R as a single memorized number rather than a family of values matched to units.
What to do instead
Convert every temperature to kelvin before substitution (T = t + 273.15). Choose R to match your units: 8.314 J/(mol·K) with pascals and cubic meters, or 0.0821 L·atm/(mol·K) with liters and atmospheres. Check that the units cancel to the units of the quantity you are solving for.
Confusing the first and second laws of thermodynamics, or assuming that an isothermal process has no heat exchange and an adiabatic process has constant temperature.
The two processes are described by similar-sounding words, and candidates memorize 'adiabatic means no heat' without connecting it to the first law ΔU = Q − W. They then misapply the ideal-gas relation.
What to do instead
Anchor each process to its defining constraint: isothermal means T constant, so ΔU = 0 and Q = W; adiabatic means Q = 0, so ΔU = −W and T changes. State the constraint in words before choosing an equation, and sketch the process on a p–V diagram to check whether work is positive or negative.
Reporting an answer with more significant figures than the given data supports, or failing to propagate uncertainty when multiplying or dividing measured quantities.
Calculators display many digits and candidates copy them verbatim. Significant-figure rules for multiplication versus addition are often confused, and uncertainty propagation is treated as an afterthought.
What to do instead
Apply the rule by operation: for multiplication and division, keep the smallest number of significant figures among the inputs; for addition and subtraction, keep the smallest number of decimal places. For uncertainties, add absolute uncertainties when adding or subtracting, and add fractional (percentage) uncertainties when multiplying or dividing. Round the final answer only, and state the uncertainty with the same decimal place as the value.