Common physics mistakes in NEET, and how to stop making them

Ask any candidate who has checked a NEET mock against the answer key and they will describe the same feeling: "I knew that." A large share of lost physics marks sit in questions the candidate could solve, but solved wrongly. With +4 for a right answer and −1 for a wrong one, each slip costs 5 marks compared with getting it right.

The good news is that these slips repeat. The same few mistakes turn up again and again, so a short list of fixes, practised until it is automatic, protects a lot of marks. This guide goes through them with a worked example each.

1. Unit prefixes and conversions

The slip. Substituting 10 μF as 10, 36 km/h as 36, or 20 cm as 20 in an SI formula.

Example. A 10 μF capacitor is charged to 100 V. Find the stored energy.

• Wrong: ½ × 10 × 100² = 50,000 J.
• Right: ½ × (10 × 10⁻⁶) × 100² = ½ × 10⁻⁵ × 10⁴ = 0.05 J.

The wrong answer is a million times too large, and it may well be one of the options, because examiners know this slip.

The fix. On your rough sheet, write every given in SI before you touch a formula. Learn the common conversions: 1 km/h = 5/18 m/s, 1 μ = 10⁻⁶, 1 n = 10⁻⁹, 1 Å = 10⁻¹⁰ m, 1 eV = 1.6 × 10⁻¹⁹ J.

2. Sign conventions in optics

The slip. Putting the object distance in as a positive number, or forgetting that a concave lens has a negative focal length.

NCERT uses the Cartesian convention: distances are measured from the pole or optical centre, positive in the direction of incident light (usually to the right), negative against it. A real object on the left always has a negative u.

Example. An object is 30 cm in front of a concave lens of focal length 20 cm. Find the image.

• Signs: u = −30 cm, f = −20 cm.
• Lens formula: 1/v − 1/u = 1/f, so 1/v = 1/f + 1/u = −1/20 − 1/30 = −(3 + 2)/60 = −1/12.
• v = −12 cm: a virtual image on the same side as the object, as a concave lens always gives for a real object.
• If u were entered as +30, the result would be v = −60 cm, which is wrong.

The fix. Draw a rough ray diagram first. If your answer says a concave lens formed a real image of a real object, you have a sign error.

3. The first-law sign convention

The slip. Mixing the physics and chemistry forms of the first law.

• NCERT physics: ΔQ = ΔU + ΔW, where ΔW is the work done by the gas.
• NCERT chemistry: ΔU = q + w, where w is the work done on the system.

Both say the same thing, but the sign of work flips between them.

Example. A gas absorbs 500 J of heat and does 200 J of work on its surroundings. Find ΔU.

• Physics form: 500 = ΔU + 200, so ΔU = 300 J.
• Chemistry form: q = +500 J, w = −200 J (work done by the system), so ΔU = +300 J. Same answer.

The fix. Pick one convention per subject and write it at the top of your rough work every time. For an ideal gas, ΔU depends only on temperature, so ΔU = 0 in any isothermal process.

4. Using a formula outside its conditions

Every formula has a condition. The slip is forgetting it.

Example. Find g at a height equal to Earth's radius R.

• Using the approximation: g(1 − 2R/R) = −g. Clearly absurd.
• Using the exact form: g(h) = g R²/(R + h)² = g R²/(2R)² = g/4.

The fix. Learn each formula together with its condition, as the kinematics and gravitation guides set them out.

5. Forgetting the internal resistance

The slip. Assuming the terminal voltage of a cell equals its emf.

Example. A cell of emf 12 V and internal resistance 1 Ω drives a 5 Ω resistor. Find the terminal voltage.

• I = E/(R + r) = 12/6 = 2 A.
• Terminal voltage V = E − Ir = 12 − 2 × 1 = 10 V, not 12 V.
• While a cell is being charged, the current reverses and V = E + Ir.

The fix. Whenever a question mentions r, write V = E − Ir before anything else.

6. Adding vectors as if they were numbers

The slip. Adding a 3 N force and a 4 N force to get 7 N when they act at an angle.

• At right angles, the resultant is √(3² + 4²) = 5 N.
• In general, R = √(A² + B² + 2AB cos θ). 7 N is the answer only when θ = 0.

The same slip appears with relative velocity, with electric fields from two charges, and with magnetic fields from two wires.

7. Mixing up series and parallel rules

Capacitors combine the opposite way to resistors, and springs combine like capacitors. Write this table once a week until it is automatic.

8. Misreading graphs and question stems

• Graphs: read both axes and their units before the curve. The slope of a v–t graph is acceleration; the area under it is displacement. A graph of V₀ against ν and a graph of V₀ against 1/λ look similar but have different slopes.
• Stems: "Which of these is NOT correct?" and "the ratio of B to A" are where careful candidates lose marks. Underline NOT, EXCEPT, INCORRECT and the order of a ratio on the question paper itself.
• Time: spending four minutes on one question is a mistake too. If a question has not yielded in about two minutes, mark it and move on.

Practice: find the answer, avoid the trap

1. A 2 kg body moves at 36 km/h. Its kinetic energy is: (a) 36 J (b) 100 J (c) 1,296 J (d) 72 J
2. An object is 15 cm in front of a concave mirror of focal length 10 cm. The image is at: (a) 30 cm in front of the mirror (b) 30 cm behind it (c) 6 cm in front (d) 6 cm behind
3. Two identical springs of constant 200 N/m are joined in series. The combined constant is: (a) 400 N/m (b) 200 N/m (c) 100 N/m (d) 50 N/m
4. Capacitors of 3 μF and 6 μF are in series. The equivalent capacitance is: (a) 9 μF (b) 2 μF (c) 4.5 μF (d) 18 μF
5. An ideal gas is compressed isothermally. The change in its internal energy is: (a) positive (b) negative (c) zero (d) equal to the work done
6. The value of g at a depth R/2 below Earth's surface is: (a) g/4 (b) g/2 (c) 3g/4 (d) 2g
7. A cell of emf 6 V and internal resistance 0.5 Ω is connected to a 2.5 Ω resistor. The terminal voltage is: (a) 6 V (b) 5 V (c) 4 V (d) 1 V
8. Forces of 6 N and 8 N act on a body at right angles. The resultant is: (a) 14 N (b) 2 N (c) 10 N (d) 48 N

Answers

1. (b) 36 km/h = 10 m/s; ½ × 2 × 10² = 100 J. (c) is the unconverted trap.
2. (a) u = −15, f = −10: 1/v = 1/f − 1/u = −1/10 + 1/15 = −1/30, so v = −30 cm, a real image in front.
3. (c) 1/k = 1/200 + 1/200 = 1/100.
4. (b) 3 × 6/(3 + 6) = 18/9 = 2 μF.
5. (c) For an ideal gas, U depends only on T, which is constant.
6. (b) g(d) = g(1 − d/R) = g(1 − ½).
7. (b) I = 6/3 = 2 A; V = 6 − 2 × 0.5 = 5 V.
8. (c) √(36 + 64) = 10 N.

What to do next

• Start an error log with three columns: question, what you did, the category from this guide.
• After every mock, count the errors in each category. Fix the largest category first.
• Before each mock, read the table of formula conditions in section 4 and the series–parallel table in section 7.
• Revise the full set of formulas with the NEET physics formula sheet.