10 Silly Physics Mistakes That Cost Students Marks (and How to Fix Them)

Students who score well in physics usually don't know much more physics than everyone else. They make fewer avoidable mistakes. Here are the 10 most common — check yourself against every one.

1. Forgetting to convert units

• cm → m
• g → kg
• µF → F
• kWh → J

If you plug 15 cm into F = kq_1 q_2 / r^2 without converting, your answer is off by a factor of 10⁴. Rule: always convert to SI before substituting.

2. Sign convention in optics

Mirror formula: 1/f = 1/v + 1/u. Both v and u are typically negative for real objects/images in front of a concave mirror. Get the sign wrong once and the whole answer is wrong.

Fix: always draw a diagram first. Object left, incident light going right. Distances left of mirror are negative.

3. Applying Coulomb's law with signed charges

F = kq_1 q_2 / r^2 — if you plug signed values, you get a signed force which is confusing. Use magnitudes for the force size, then decide attraction/repulsion by sign of q_1 q_2 separately.

4. Wrong number of significant figures

Board exams expect answers to 2–3 significant figures. Writing F = 1.19835 N looks smart but loses a mark. Use F ≈ 1.20 N — physically meaningful precision.

5. Missing "+ C" in integration

Class 12 physics uses indefinite integrals in EMF, work-energy problems. Forgetting + C doesn't matter if you evaluate definite integrals — but if the answer is an antiderivative, + C is worth a mark.

6. Ignoring direction in vector problems

Electric field, force, velocity — these are vectors. Writing E = 5 N/C is incomplete. Must specify direction (e.g., along +x axis) or draw the vector.

7. Wrong formula for series vs parallel

• Resistors in series: R = R_1 + R_2
• Resistors in parallel: 1/R = 1/R_1 + 1/R_2
• Capacitors in series: 1/C = 1/C_1 + 1/C_2 — like parallel resistors!
• Capacitors in parallel: C = C_1 + C_2 — like series resistors!

Trap: capacitors and resistors have OPPOSITE formulas. Confusing them is worth −2 marks easily.

8. Confusing angular velocity with frequency

\omega = 2\pi f. Some students use \omega = f and get wrong SHM answers. Radians are involved, so 2π matters.

9. Dropping the negative sign in EMF

Faraday's law: \varepsilon = -\frac{d\Phi}{dt}. The minus sign (Lenz's law) is physical — it tells direction. Dropping it in a derivation costs a mark; missing it in a conceptual question costs 3.

10. Using degrees when the formula expects radians

\sin\theta in a calculator gives one answer in degree mode and another in radian mode. Physics formulas usually assume radians. Fix: always check your calculator's mode before starting.

The habit that fixes all 10

Before submitting any numerical:

1. Check units.
2. Check signs.
3. Check significant figures.
4. Read the question ONE more time — did you answer what was asked?

That 30-second check catches most of these mistakes before the examiner does.