NSEP common mistakes
NSEP · Understand the traps and how to avoid them.
Computing a numerical answer purely in SI base units without tracking how many significant figures the given data justify, then writing all the calculator digits (e.g., 9.80665 m/s² when g is given as 9.8).
Candidates treat significant figures as a formatting afterthought rather than a rule of measurement, and calculators hand them extra digits for free, so no mental check ever fires.
What to do instead
Before substituting numbers, write the input precision (e.g., 9.8 has 2 s.f., 0.250 has 3 s.f.) beside the equation, then round the final answer to the least precise input and keep one extra digit only in intermediate steps. Practise by re-solving five problems and deliberately reporting the wrong-sig-fig answer to see the penalty.
Using the standard kinematics equations (v = u + at, s = ut + ½at²) when acceleration is not constant, e.g., on a velocity–time graph that is curved.
These equations are drilled so heavily in Class 11 that they become the default reflex, and the candidate does not check the precondition 'constant a' before substituting.
What to do instead
Adopt a two-step gate: first classify the motion as uniform or non-uniform from the graph or the force description; if non-uniform, use calculus (v = dx/dt, a = dv/dt) or the area under the v–t curve. Build a one-page table mapping 'what is given' to 'which tool is valid' and consult it before every mechanics problem.
Drawing a free-body diagram that omits a force or misassigns its direction - typically leaving out the normal force on an inclined plane, inventing a 'centrifugal force' in the ground frame, or putting friction in the direction of motion rather than opposing relative slipping.
Candidates jump to equations with the diagram still in their head, and pseudo-forces are learned as a shortcut without the condition that they exist only in a non-inertial frame.
What to do instead
Make the FBD a mandatory written step: label every contact force (normal, friction, tension) and every long-range force (gravity), state the frame you are working in at the top of the page, and only then resolve components. For friction, explicitly write 'relative motion of the surfaces is ___ , so friction points ___' before using it.
Applying conservation of mechanical energy across a process where non-conservative forces (friction, air drag, an inelastic collision) do work, and getting an answer larger than physically possible.
Energy conservation is taught first as a clean tool, so candidates reach for it whenever a height or speed appears, forgetting the 'conservative forces only' clause.
What to do instead
Before writing KE₁ + PE₁ = KE₂ + PE₂, list the forces doing work between the two states; if any is non-conservative, switch to the work–energy theorem (W_net = ΔKE) or the full first law with the heat term. Sanity-check by asking whether the answer implies energy was created.
Solving rotational problems by plugging into τ = Iα or L = Iω with the moment of inertia about the wrong axis (e.g., using ½MR² for a disc rotating about a diameter instead of about its centre), or ignoring that a rolling body's kinetic energy has both translational and rotational terms.
The standard moments of inertia are memorised as a list without the axis attached, and rolling is treated as pure translation because the centre of mass moves in a straight line.
What to do instead
Always sketch the axis on the diagram and write the moment of inertia with its axis in words ('about the central perpendicular axis'). For rolling, write KE = ½Mv² + ½Iω² every time and use the rolling constraint v = ωR; then check the limiting cases (a hoop should have twice the rotational share of a solid disc).
Treating gravitational field and gravitational potential as interchangeable, or setting gravitational potential energy to zero at the Earth's surface when the formula used is U = −GMm/r, which is zero at infinity.
Both quantities are 'gravity', both are written with G and M, and the negative sign and the reference point are easy to skim past.
What to do instead
Keep a reference sheet pairing each quantity with its defining integral (field = −dV/dr) and its zero point. Before substituting, write 'zero of potential is at ___' and check the sign of the result against the physical situation (a bound object must have negative total energy).
Mixing up the thermodynamic sign convention - using ΔU = Q − W with work defined as work done on the gas, or the reverse - so an expansion comes out with the wrong sign for temperature change.
Textbooks and derivations use both conventions, and the candidate never fixes one convention explicitly at the start of a solution.
What to do instead
Declare the convention in writing on the first line of every thermodynamics answer ('W = work done by the gas; ΔU = Q − W'), then check with a known case: isothermal free expansion of an ideal gas must give ΔU = 0 and Q = 0. Redo two problems under the opposite convention to see where the sign flips.
Computing the resultant of two waves or two sources by adding amplitudes arithmetically when the sources are not coherent or the phase difference is not a multiple of π, instead of using superposition with the actual phase difference.
Interference is memorised as 'constructive means add, destructive means subtract', which is only true at phase differences of 0 and π, and coherence is assumed rather than checked.
What to do instead
For any superposition problem, first write the phase difference δ (from path difference, Δφ = 2πΔx/λ) and use A² = A₁² + A₂² + 2A₁A₂cos δ. State explicitly whether the sources are coherent; if not, average intensities instead of adding amplitudes.
Applying the mirror/lens formula with the wrong sign for the focal length or image distance because the Cartesian sign convention was not fixed before substitution, giving a virtual image where a real one is expected.
The formula is symmetric in appearance, so the sign lives entirely in the convention, which candidates hold loosely and inconsistently between problems.
What to do instead
Write the convention at the top of the page (distances measured from the pole/optical centre, positive in the direction of incident light) and assign signs to u, f, v before touching the formula. Verify with a ray diagram sketch: a convex lens with an object outside f must give a real, inverted image on the far side.
Leaving answers in a form that cannot be marked - a bare number with no unit, a vector given only as a magnitude, or a 'derivation' that starts from the result it is supposed to prove.
Under time pressure candidates optimise for reaching a number and skip the presentation steps that carry marks, and circular derivations feel faster because the endpoint is already known.
What to do instead
Rehearse a fixed answer template: given data with units, the principle stated in words, the symbolic equation, substitution, result with unit and direction, and a one-line physical check. For derivations, start only from definitions or previously established results and number each step so the logic is visible.