Physics
What NSEJS physics tests
The paper re-dresses the class 9–10 syllabus - motion, force, gravitation, fluids, heat, sound, ray optics and circuits - in unfamiliar settings. Two habits earn marks: draw the free-body diagram before writing any equation, and check the units or a limiting case at the end.
- Kinematics: v = u + at, s = ut + ½at², v² = u² + 2as apply only to uniform acceleration. Slope of an s–t graph is velocity; slope of a v–t graph is acceleration; area under v–t is displacement.
- Friction: f ≤ μN. Static friction adjusts itself to the applied force, kinetic friction is fixed. At the angle of repose tan θ = μ.
- Apparent weight: in a lift accelerating up, N = m(g + a); accelerating down, N = m(g − a); in free fall N = 0.
- Optics: n₁ sin i = n₂ sin r, with angles measured from the normal; n = c/v; apparent depth = real depth ÷ n.
- Circuits: series shares current, parallel shares voltage; P = I²R = V²/R.
Worked example. A 2 kg block rests on a smooth 30° incline (g = 10 m/s²). Its weight has components mg sin 30° = 10 N down the slope and mg cos 30° ≈ 17.3 N into the surface. With no friction the net force is 10 N, so a = 10/2 = 5 m/s² down the slope. Perpendicular to the slope there is no acceleration, hence N ≈ 17.3 N - not 20 N. The acceleration is mass-independent; a friction term would give a = g(sin θ − μ cos θ).
Traps. Writing N = mg on an incline; mixing g = 10 with g = 9.8 inside one solution; measuring refraction angles from the glass surface instead of the normal; assuming equal currents through parallel bulbs of different resistance; dropping the sign of g in vertical-throw problems.