Physics: Mechanics, Optics, and Electricity
JENPAS-UG Physics checks whether you can apply Class 11–12 mechanics, optics, and electricity to numerical and conceptual MCQs. Expect unit analysis, force diagrams, circuit reasoning, and ray optics rather than long derivations.
Core rules and concepts
- Dimensional homogeneity: every term in a valid equation must have the same base dimensions; use this to reject wrong formula choices.
- Newton’s second law in component form: ΣF_x = ma_x. Static friction satisfies f ≤ μN; equality holds only at impending motion.
- Work–energy theorem: W_net = ΔK. Conservative forces have potential energy; friction dissipates mechanical energy.
- Ohm’s law and resistor networks: V = IR; series R_eq = R1 + R2 + …; parallel 1/R_eq = Σ(1/R). Power P = VI = I²R = V²/R.
- Thin lens/mirror formula: 1/f = 1/v − 1/u for lenses with Cartesian sign convention; magnification m = v/u.
Worked example
A 2 kg block slides from rest down a 5 m frictionless incline at 30°. Find its speed at the bottom. The vertical drop is h = 5 sin 30° = 2.5 m. By energy conservation, mgh = ½mv², so v = √(2gh) = √(2 × 9.8 × 2.5) = √49 = 7 m/s. Mass cancels; if friction were present, subtract the work done by friction from mgh before solving.
Traps that cost marks
- Mixing sign conventions in optics: distances measured opposite the incident light are negative.
- Using g = 10 m/s² when options clearly assume 9.8 m/s².
- Assuming tension is uniform across a string with mass or over a pulley with inertia.
- Setting static friction equal to μN when the block is not slipping.
- Saying acceleration is zero at the top of a projectile’s path - vertical velocity is zero, but acceleration is g downward.