percentage uncertainty = (absolute uncertainty/measured value) x 100

“Measuring a wire as 1.20 m ± 0.02 m yields a 1.7 % uncertainty, like a recipe tolerating 5 % spice variation.”

Percentage uncertainty = (absolute uncertainty ÷ measured value) ×100, valid only for a non-zero measured value after any systematic correction. The common trap is to use the raw reading or ignore that relative uncertainties, not percentages, add when quantities are multiplied or divided.

percentage uncertainty = (absolute uncertainty/measured value) x 100

Memory trick: Δ% = (Δ/Value)×100; never divide by zero.

R = u²sin2θ/g

“A cricket bowler’s ball lands at the same height; its horizontal range R is given by R = u² sin2θ / g.”

From the horizontal and vertical components, R = u² sin2θ / g assumes level launch and landing and ignores air resistance. It is valid only for constant g on level ground. Forgetting the equal-height condition inflates the predicted range.

R = u²sin2θ/g

Memory trick: R = (u²/g)·sin2θ → ‘Sin twice the angle’

energy vs power

“A 60 W bulb consumes 60 J each second, while a 1200-J battery can keep it lit for 20 s, showing stored energy vs usage rate.”

Energy (J) is the total capacity to do work; power (W) is the rate of energy transfer, P = dE/dt. Use average power ΔE/Δt for constant cases and instantaneous dE/dt for varying ones. The trap: applying E = P·t when power changes, instead of integrating the power curve.

Memory trick: Energy = joules; Power = joules / second

Significant figures — common mistake

“Like a chef tasting a sauce with a pinch of salt before plating, you keep extra ‘guard’ digits during calculations, not the final garnish.”

When multiplying or dividing, the answer keeps the same number of significant figures as the factor with the fewest. For addition/subtraction, match the least number of decimal places. Keep extra guard digits throughout the work and round only at the final step; rounding early can lower precision and give a wrong sig-fig count. Exact counted numbers and defined constants have unlimited significant figures.

Memory trick: Guard digits now, round at the end.

Newton's second law

“When a rocket fires, the thrust accelerates it, but the increasing fuel loss means the same thrust gives higher acceleration later.”

Newton’s second law states that the vector sum of external forces equals mass times acceleration (F = ma) and the direction of a is the same as the net force. For systems with changing mass, the correct form is F = d(mv)/dt, but for constant-mass problems use F = ma. A frequent mistake is to treat F and a as scalars and ignore direction.

Memory trick: F vector = m × a vector; same direction.

Friction in rolling

“A rolling hoop on a smooth floor feels a gentle static-friction push, like a hand turning a knob without slipping.”

Static friction acts at the contact in pure rolling, providing the torque τ = fR while doing no work because the contact point is instantaneously at rest. The exam trap is to assume kinetic friction or energy loss; only when slipping starts does friction dissipate energy.

Memory trick: Static friction = torque, no work → energy conserved

Variation of g

“A hiker on a 5 km hill feels a tiny loss of weight, just as a ball dropped from a tall building falls slightly slower.”

Outside Earth, g at altitude h is g = g0·(R/(R+h))² ≈ g0(1−2h/R) for h≪R. Inside a uniform Earth, at depth d below the surface g = g0·(1−d/R) (or g = g0·r/R with r measured from the centre). The trap: applying the exterior inverse-square law inside the Earth is wrong; the interior variation is linear.

Memory trick: Outside: inverse-square; Inside: linear drop → g = g0·(1−d/R).

Physical pendulum

“A door swinging on its hinges behaves like a physical pendulum, not a simple bob.”

Period T = 2π√(I/(m g d)), where I is about the pivot and d is distance from pivot to CM. The trap: using T=2π√(l/g) with l as door length; only works if I = m d² (point mass). Also keep the small-angle approximation (θ≲10°).

Memory trick: T = 2π√(I/(m·g·d))

Doppler effect — common mistake

“When a train whistles, some students write a minus sign for both source and observer, forgetting the pitch actually rises while approaching.”

Mistaking the sign convention gives a lower frequency for an approaching source. Correct: source toward observer → denominator (v-vs) makes f' larger; observer toward source → numerator (v+vo) also raises f'. Trap: mixing up which motion belongs to numerator or denominator.

Memory trick: Source toward → denominator -; observer toward → numerator +

Isothermal versus adiabatic

“A gas in a well-insulated cylinder (adiabatic) heats up when compressed, unlike the same gas in a water bath (isothermal) that stays at constant temperature.”

Isothermal: T constant, ΔU=0 for ideal gas, so Q = W and PV = const. Adiabatic: Q=0, so ΔU = –W and temperature changes, obeying PV^γ = const. The trap is mixing up PV=const with PV^γ=const—heat exchange decides which law applies.

Memory trick: Isothermal: Q↔W, ΔU=0; Adiabatic: Q=0, ΔU=−W, PV^γ=const

Capacitance and dielectrics

“Placing a rubber sheet between the plates of a capacitor is like slipping a sponge into a water tank – it holds more water (charge) for the same height (voltage).”

With dielectric constant K, C = K ε₀A/d, so C increases by K and the internal field becomes E/K. For fixed charge, V=Q/C falls by 1/K; for fixed voltage, Q=CV rises by K. Trap: using C=ε₀A/d without K or assuming energy stays same when a dielectric is inserted.

Memory trick: Dielectric: C×K, E÷K, V÷K (Q const) or Q×K (V const).

Meter bridge and potentiometer — common mistake

“Like a speedometer that lags because of friction, a voltmeter loads the circuit and under-reports the cell’s emf.”

A voltmeter has finite internal resistance, so it draws current and the terminal voltage falls below the emf. The mistake is assuming its reading equals the emf; ensure the voltmeter’s resistance is much larger than the circuit’s.

Memory trick: Voltmeter loads → lower reading

Ampere's circuital law — common mistake

“Like a river flowing steadily between fixed banks, Ampère’s law holds only for steady currents, not when the electric field’s level changes.”

∮B·dl = μ0 I_enc is exact for time-independent currents. If the electric flux changes, the displacement-current term μ0ε0 dΦE/dt must be added (Maxwell–Ampère). The shortcut B = μ0 I/(2πr) comes from cylindrical symmetry of an infinite straight wire; it fails for a solenoid, whose field is μ0 n I inside. The exam trap is applying the straight-wire formula to any long coil.

Memory trick: Steady → no dE/dt; symmetry decides which B formula works.

Reactance and impedance — common mistake

“Mixing orange juice and water makes a new drink, but adding R, X_L and X_C arithmetically gives a wrong circuit ‘taste’.”

Impedance is a vector: resistance lies on the real axis, reactance on the imaginary axis, so Z=R+j(X_L−X_C). Its magnitude is √(R²+(X_L−X_C)²). The classic mistake is treating the three quantities as simple scalars, which yields an incorrect magnitude and phase angle.

Memory trick: Think Z as a right-triangle: legs R and (X_L−X_C)

Single-slit diffraction

“A narrow hallway acts like a single slit, spreading a flashlight beam into a wide central glow.”

Minima occur when a sinθ = nλ (n=1,2,…). The central maximum width ≈ 2λ/a, so a narrower slit makes a broader bright region. A common trap is using n=0 for minima or mixing up the condition for maxima (which lie midway between minima).

Memory trick: Minima: a sinθ = nλ, start n=1

Light-emitting diode

“A red LED glows because its semiconductor bandgap matches the energy of red photons, just as a piano string vibrates at a specific note.”

LED color is set by the direct-bandgap energy; only direct-gap semiconductors emit light efficiently. Forward voltage ≈ Eg/q plus a small drop. Trap: assuming any diode (e.g., Si) will shine visibly—indirect gaps give negligible light.

Memory trick: LED color ↔ bandgap (Eg≈q·Vf)