Physics: mechanics, fluids, electricity, optics and thermodynamics
Physics tests model selection and unit discipline across mechanics, fluids, electricity, optics and thermodynamics. Expect force diagrams, conservation laws, circuit analysis, lens calculations and gas-law reasoning.
Core rules
- Mechanics: Newton's second law ΣF = ma; kinetic energy ½mv²; gravitational potential energy mgh; momentum conserved in collisions when external impulses are negligible.
- Fluids: continuity A1v1 = A2v2; Bernoulli p + ½ρv² + ρgh = constant for ideal flow; Poiseuille flow depends on radius to the fourth power; Archimedes' buoyant force equals the weight of displaced fluid.
- Electricity: Ohm's law V = IR; power P = VI; series resistances add, parallel conductances add; Kirchhoff's junction and loop rules.
- Optics: Snell's law n1 sin θ1 = n2 sin θ2; thin lens 1/f = 1/do + 1/di; magnification m = −di/do.
- Thermodynamics: ideal gas PV = nRT; first law ΔU = Q − W; Carnot efficiency = 1 − Tc/Th with absolute temperatures.
Worked example
Water flows in a horizontal pipe from area 10 cm² at 1.0 m/s to area 2.5 cm². Find the speed and pressure drop (ρ = 1000 kg/m³). Continuity: A1v1 = A2v2, so v2 = (10/2.5) × 1.0 = 4.0 m/s. Bernoulli with equal heights: p1 − p2 = ½ρ(v2² − v1²) = ½ × 1000 × (16 − 1) = 7500 Pa. Reasoning: the narrower section has higher speed and lower pressure; pressure falls by 7.5 kPa.
Traps that cost marks
- Mixing cm² and m² in continuity.
- Using gauge and absolute pressures interchangeably.
- Adding parallel resistances as if in series.
- Forgetting absolute temperatures in Carnot efficiency.
- Ignoring sign conventions in thin-lens calculations.