AAPT NCPT (Level 1) common mistakes
AAPT NCPT (Level 1) · Understand the traps and how to avoid them.
Applying conservation of mechanical energy to a block sliding with friction without adding the negative work done by friction, so the final kinetic energy is set equal to the initial potential energy.
Students learn energy conservation first in the frictionless case and treat it as a universal law, forgetting that non-conservative forces change total mechanical energy.
What to do instead
Before writing any energy equation, list every force doing work and classify it as conservative or non-conservative; then write K_i + U_i + W_nc = K_f + U_f and compute W_nc = -f_k·d explicitly.
Treating a two-dimensional collision or projectile problem as one-dimensional, setting the total initial momentum equal to a scalar final momentum without resolving components.
Vectors are introduced early and students revert to scalar arithmetic under time pressure, especially when the problem statement gives only magnitudes.
What to do instead
Adopt a fixed routine: draw axes, write p_x and p_y conservation as two separate equations, and only combine them with Pythagoras at the very end if a magnitude is requested.
Using the initial temperature instead of the change in temperature (or mixing Celsius and Kelvin) when applying Q = mcΔT or the ideal gas law.
The formulas look simple, so students substitute numbers without tracking units; ΔT in Celsius equals ΔT in Kelvin but T itself does not.
What to do instead
Convert all temperatures to kelvin at the start of the problem and label every symbol with units before substituting; for ΔT, write T_f - T_i explicitly rather than a bare number.
Assuming a process is reversible and applying ΔS = Q/T to a spontaneous irreversible process such as free expansion or heat flow across a finite temperature difference.
The reversible formula is the only entropy relation most students memorise, and they do not check whether the actual path is reversible.
What to do instead
For any entropy change, construct a reversible path between the same initial and final states and compute the integral along that path; for free expansion use ΔS = nR ln(V_f/V_i), not Q/T = 0.
Applying the thin-lens or mirror equation with a sign convention chosen inconsistently, for example treating a virtual image distance as positive in one step and negative in the next.
Different textbooks use different sign conventions, and students blend them without committing to one system.
What to do instead
State the sign convention at the top of the solution (e.g. real-is-positive for object and image distances, focal length negative for diverging lenses) and check every substituted value against it before solving.
Predicting the direction of a magnetic force on a moving charge or current-carrying wire using the direction of B rather than the cross product v × B, giving an answer off by 90 degrees.
The right-hand rule has three vectors and students confuse which finger represents which quantity, especially when the velocity is not perpendicular to the field.
What to do instead
Use the flat-hand version: fingers point along v, curl toward B, thumb gives F for a positive charge; then verify the result is perpendicular to both v and B and reverse it for a negative charge.
Adding resistor values in parallel or capacitor values in series by direct summation, i.e. treating the reciprocal rule as a simple sum.
Series and parallel rules are memorised as paired formulas and the reciprocal structure of the parallel resistor and series capacitor rules is easily swapped under exam pressure.
What to do instead
Derive the rule once from the shared quantity (same current for series, same voltage for parallel) and write the governing physical constraint next to each calculation; sanity-check that a parallel combination is smaller than the smallest resistor.
Calculating a photoelectric-effect threshold or photon energy in electronvolts but substituting the work function in joules (or vice versa) without converting.
The constants h and c are often quoted in eV·s and eV·nm, and students mix unit systems within a single equation.
What to do instead
Pick one energy unit for the whole problem, write the conversion factor (1 eV = 1.60 × 10^-19 J) beside the equation, and confirm the final kinetic energy is positive and physically sensible.
Reporting a measured quantity with more significant figures than the instrument resolution supports, or omitting units and uncertainty in a final lab result.
Calculators return many digits and students copy them verbatim without applying the rules for propagation of uncertainty.
What to do instead
Record the resolution of each instrument before measuring, propagate uncertainty through the calculation, and round the final answer to the same decimal place as its absolute uncertainty, stating units explicitly.
Plotting raw data and fitting a curve by eye instead of linearising the relationship to obtain a straight-line graph whose slope yields the physical constant.
Students are used to graphing y versus x directly and do not recognise that a power law such as T = 2π√(L/g) must be squared or logged to give a linear plot.
What to do instead
Before plotting, rearrange the theoretical relation into y = mx + c form, identify what to plot on each axis, and state in advance which slope or intercept corresponds to the quantity being measured.