Traps

How NDA loses you marks even when you know the formula

Physics distractors are about CONFUSED FORMULA APPLICATION — wrong formula picked, right formula misapplied (sign, unit, direction), or one missing term in a multi-step setup. Different from Maths' factor-of-2 / sign-flip numeric cells and English's near-synonym semantic shapes. Each trap below is illustrated on a real PYQ.

trap shapes: 11
skill strands affected: 3
playbooks per top trap: 2
worked examples below: 10

How to use this page

Read once cover-to-cover. Then re-read the strand relevant to your next practice session — the trap is far easier to spot when you’ve just been primed on its mechanism. NDA recycles these same shapes year after year; pattern recognition pays.

Recall traps (Sound · Modern · Astronomy · Energy · Units)

Specific heat — 'depends on mass and shape' wrong option

Affects: Heat and Thermodynamics

The mechanic

Specific heat is an INTENSIVE property — it depends on the material only, not on the mass or the shape of the body. But the wrong options often list 'depends on mass' or 'depends on shape' (paraphrased) and a hurried student picks the first plausible-sounding one. The right answer is 'independent of mass and shape; depends on temperature for some materials.'

The fix

On any statement-truth question about specific heat, mentally tag each option: 'intensive (depends on material only)?' — and reject anything claiming mass/shape dependence. The same intensive-vs-extensive lens cuts traps on density, refractive index, resistivity.

Worked example from the bank

Example 1Heat and ThermodynamicsHARD

Which of the following statements about specific heat of a body is/are correct? 1. It depends upon mass and shape of the body. 2. It is independent of mass and shape of the body. 3. It depends only upon the temperature of the body. Select the correct answer using the code given below:

[Q101 · Apr · 2018]

Scientist–discovery pair swap

Affects: Modern Physics

The mechanic

The match-list format shuffles famous scientists with the wrong discoveries — Chadwick=photoelectric, Einstein=neutron, Marie Curie=relativity. One pair is correct, the other 2–3 are deliberately swapped. The trap relies on you trusting a single recognised name as 'right' without checking all the pairs.

The fix

Read EACH pair against your memory. If even one pair is wrong, the whole option is wrong (NDA format). Memorise the 8 canonical pairs cold: Chadwick=neutron, Einstein=photoelectric, Marie Curie=radium/polonium, Rutherford=nuclear model, Bohr=atomic model, Planck=quantum, J.J. Thomson=electron, Roentgen=X-rays.

Apply traps (Light · Mechanics · WEP · Gravity · SHM)

Mirror / lens sign convention flip

Affects: Light and Optics

The mechanic

Cartesian sign convention: distances measured from the pole, +x to the right of the incident light. Object distance u is NEGATIVE for a real object on the left. f is negative for concave lens / convex mirror. Mirror formula 1/v + 1/u = 1/f vs lens formula 1/v − 1/u = 1/f. Get any sign wrong and v lands on the wrong side, sometimes with the wrong magnitude.

The fix

ALWAYS draw a ray diagram first, then write down the signs of u and f. Plug in with sign included. Check whether v ends up + or − and translate: + = same side as outgoing light (real for mirror, virtual for lens); − = opposite side (virtual for mirror, real for lens — but in NDA's convention this distinction is rare).

Worked example from the bank

Example 1Light and OpticsHARD

For a human eye, where u is the distance of an object from the eye, f is the focal length of the lens and v is the distance of image from the eye, which is the correct schematic graph ?

[Q128 · Apr · 2024]

Total internal reflection in the wrong direction

Affects: Light and Optics

The mechanic

TIR only happens when light goes from a DENSER medium to a RARER one (e.g. water→air, glass→air). Options in TIR questions often offer the reverse (rare→dense) which is structurally impossible — but in a setup with multiple medium boundaries the eye loses track of which surface is which. The critical angle is for the denser side.

The fix

On any TIR question, label every medium with its μ. Draw the ray. Confirm the suspect-TIR surface has μ_above < μ_below. If not, the trap is asking you to invoke TIR where it can't physically occur.

Worked example from the bank

Example 1Light and OpticsHARD

A glass slab (refractive index 1.5) is cut as parallelogram ABCD with BC polished as a reflector. Light is incident at AB from inside at

\pi/3

. Which angle

ABC = \theta

causes reflected light to retrace its path?

[Q74 · Apr · 2026]

Mass doesn't appear in pendulum / free-fall period

Affects: Oscillations and Waves, Gravitation

The mechanic

T = 2π√(L/g) has no m. v_freefall = √(2gh) has no m. v_escape = √(2gR) has no m. The trap is to vary mass in the problem and offer 'T doubles when mass doubles' as an option. Pendulum period depends ONLY on length and g.

The fix

Write the formula symbol-by-symbol before plugging numbers. If m isn't in the formula, changes in m do NOTHING. Same lever for free-fall time, projectile range (without air drag), and orbital period.

Worked example from the bank

Example 1Oscillations and WavesMODERATE

A simple pendulum having bob of mass

m

and length of string

l

has time period of

T

. If the mass of the bob is doubled and the length of the string is halved, then the time period of this pendulum will be

[Q135 · Sep · 2022]

CGS / SI unit-system mix

Affects: Units, Measurement and Dimensions, Fluid Mechanics and Properties of Matter

The mechanic

A question gives pressure in mm Hg and density in g/cm³, asks for height in m. Mixing CGS and SI mid-calc shifts every answer by powers of 10. NDA tests this directly: 1 dyne = 10⁻⁵ N, 1 erg = 10⁻⁷ J, 1 poise = 0.1 Pa·s — distractors are off by exactly the cgs↔SI multiplier.

The fix

Convert every quantity to SI before computing. If the question is dimensional ('1 dyne equals'), recall the conversion: F = M·L·T⁻² so 1 g·cm·s⁻² = 10⁻³·10⁻² = 10⁻⁵ kg·m·s⁻² = 10⁻⁵ N.

Worked example from the bank

Example 1Units, Measurement and DimensionsMODERATE

1 dyne (a unit of force in CGS system) equals to

[Q113 · Apr · 2019]

Reason traps (E&M · Heat · Fluid Mechanics)

Heat in parallel vs series — wrong ratio direction

Affects: Electricity and Magnetism

The mechanic

Two equal resistors R, same V across the combination. In SERIES, R_total = 2R and P = V²/2R. In PARALLEL, R_total = R/2 and P = 2V²/R. Ratio P_parallel / P_series = (2V²/R) / (V²/2R) = 4. The trap option is 1/4 (right magnitude, inverted ratio) — picking it loses you the question because you forgot which way the ratio runs.

The fix

Always write the formula for the LARGER quantity first. 'Parallel has SMALLER R_total, so MORE current, so MORE heat for given V.' Parallel wins on heat dissipation at same V; series wins at same I. Tag which scenario the question is asking.

Worked example from the bank

Example 1Electricity and MagnetismHARD

Two conducting wires of the same material and of equal lengths and equal diameters are first connected in parallel and then in series in a circuit across the same potential difference. The ratio of heat produced in parallel and series combinations is

[Q128 · Apr · 2025]

Calorimetry — forgetting a latent heat term

Affects: Heat and Thermodynamics

The mechanic

Ice at −10°C added to water at 30°C, find final T. Setup needs THREE heat exchanges: ice warming −10→0 (sensible, mc·10), ice melting at 0 (latent, mL_f), water cooling 30→T (sensible, mc·ΔT). Skip the latent term and you get a final T that's 10–20°C off — and there's almost always an answer option that matches the no-latent calculation.

The fix

Map out the temperature journey for EACH substance before writing an equation. Cross any phase boundary (0°C for water-ice, 100°C for water-steam) and you owe a Q=mL term. Set ∑Q_gained = ∑Q_lost. The final state (all liquid? mixed? all solid?) is part of the setup, not an output.

Worked example from the bank

Example 1Heat and ThermodynamicsHARD

A 5 g piece of ice at

-20\,°C

is put into

m

kg of water at

30\,°C

. The final temperature is

0\,°C

in liquid phase. What is the value of

m

in kg?

[Q61 · Apr · 2026]

Density mixing — arithmetic vs harmonic mean confusion

Affects: Fluid Mechanics and Properties of Matter

The mechanic

Two substances of densities ρ₁ and ρ₂ mixed in EQUAL VOLUME: ρ_avg = (ρ₁+ρ₂)/2 (arithmetic mean). Mixed in EQUAL MASS: ρ_avg = 2ρ₁ρ₂/(ρ₁+ρ₂) (harmonic mean). The harmonic mean is always smaller than the arithmetic. Distractor options swap the two formulas; the student writes the right symbols but for the wrong scenario.

The fix

Equal-volume = arithmetic mean (volumes add cleanly, so densities average). Equal-mass = harmonic mean (volumes differ, the lighter one takes up more, dragging the average down). Both formulas can be re-derived in 30 seconds from ρ = m_total / V_total.

Worked example from the bank

Example 1Fluid Mechanics and Properties of MatterHARD

Two substances of densities

\rho_1

and

\rho_2

are mixed in equal volume and their relative density is 4. When they are mixed in equal masses, relative density is 3. The values of

\rho_1

and

\rho_2

respectively are

[Q65 · Sep · 2019]

Planet-scaling ratio — inverting the wrong term

Affects: Gravitation

The mechanic

Planet has R = R_earth/2 and density 4× Earth's. Find escape velocity. M = ρV ⟹ M = 4ρ · ½·V = ½M_earth (since V scales as R³, halved-R is V_earth/8). v_esc = √(2GM/R) = √(2G·½M_e / ½R_e) = √(2GM_e/R_e) = same as Earth. The trap is to keep ρ-factor and R-factor separate and miss that M scales NOT linearly with ρ — it scales as ρ·R³.

The fix

Write M = ρV = ρ·(4/3)πR³ symbol-by-symbol. Plug ratios in for ρ AND R BEFORE computing v_esc. Combine the powers carefully: v_esc ∝ √(M/R) ∝ √(ρR²). Now apply scaling: √(4·(½)²) = √(4·¼) = 1. Same v_esc.

Worked example from the bank

Example 1GravitationHARD

Escape speed from the Earth is close to 11.2 km

s^{-1}

. On another planet whose radius is half of the Earth's radius and whose mass density is four times that of the Earth, the escape speed in km

s^{-1}

will be close to :

[Q84 · Apr · 2024]

Process variant — applying PV=const where PVⁿ=const holds

Affects: Heat and Thermodynamics

The mechanic

Isothermal process: PV = const (Boyle). Adiabatic: PVᵞ = const (γ = Cp/Cv). The question stipulates PV² = const (a non-standard polytropic process), then asks for the T₁/T₂ vs V₁/V₂ relation. Default-thinking 'PV is constant so T is constant' is wrong — only for genuine isothermal. With PVⁿ = const and PV = nRT, T·V^(n−1) = const.

The fix

Read the process specification CAREFULLY before invoking a memorised relation. If the exponent on V is anything other than 0 (isobaric), 1 (isothermal), or γ (adiabatic), derive from PV = nRT plus the given constraint. Don't assume isothermal just because P and V both appear.

Worked example from the bank

Example 1Heat and ThermodynamicsHARD

In a certain process,

PV^2 = \text{constant}

for an ideal gas. If initial temperature is

T_1

, final temperature

T_2

, initial volume

V_1

, final volume

V_2

, then which one is correct?

[Q54 · Apr · 2026]

The time-budgeted verification habit

Verification quality scales with the time you have. Pick the deepest check the budget allows — don’t skip verification entirely.

15 seconds left (Recall)

Unit + dimension check

Does the answer have the right unit? Force in Newtons not Joules; energy in Joules not Watts; light year is distance not time.

30 seconds left (Apply)

Sign + symbol check

Sign convention applied consistently? (u negative for real object in Cartesian.) Did mass appear where it shouldn’t (pendulum, free-fall)?

60 seconds left (Reason)

Full setup re-check

Heat-balance accounts for EACH phase boundary? Resistor reduction starts from innermost combination? Density-mixing formula matches equal-volume vs equal-mass?

The habit, not the rule. A 10-second verification per question recovers more marks per paper than learning a new formula — the trap is what loses students who already know the formula.