Physics - Mechanics, Fluids, Electricity, Waves and Medical Applications
The physics section tests mechanics, fluid statics and dynamics, electricity and waves, always with a medical slant: infusion pressure, blood flow, hearing, imaging and corrective lenses. Questions are short calculations in which units decide the answer.
Core rules
- Mechanics. v = v₀ + at and s = v₀t + ½at²; use energy conservation only when no non-conservative force acts. Momentum is conserved in all collisions; kinetic energy only in elastic ones.
- Fluid statics. P = P₀ + ρgh; gauge pressure omits P₀. A fluid at rest exerts pressure isotropically, so depth alone sets the pressure.
- Flow. Continuity A₁v₁ = A₂v₂ and Bernoulli along a streamline. Poiseuille: Q = πΔPr⁴/(8ηL), so flow scales with the fourth power of the radius.
- Electricity. V = IR, P = VI; resistors add in series and add reciprocally in parallel; capacitors behave in the opposite way.
- Waves and optics. v = fλ; for a thin lens 1/f = 1/d₀ + 1/dᵢ, with dᵢ negative for a virtual image and m = −dᵢ/d₀.
Worked example
An IV bag sits 1.2 m above the needle; the fluid is dilute saline, ρ ≈ 1000 kg/m³. Gauge pressure at the needle = ρgh = 1000 × 9.8 × 1.2 ≈ 11 800 Pa ≈ 11.8 kPa, about 88 mmHg (1 mmHg ≈ 133 Pa). Venous pressure is roughly 5–10 mmHg, so a favourable gradient drives the infusion; doubling the height roughly doubles the driving pressure. Note also that a 20% narrowing of a vessel leaves 0.8⁴ ≈ 0.41 of the original flow.
Traps
Mixing cm with m; confusing radius with diameter inside r⁴; reporting absolute instead of gauge pressure; sign errors in the lens equation.