Physics: Mechanics, Fluids, Circuits and Optics
Scope
The physics paper is applied and numerical: kinematics and Newton's laws, work and energy, fluids, simple circuits, waves and geometric optics, and basic thermal physics. Correct units matter as much as correct formulas.
Core rules
- Kinematics: v = u + at, s = ut + ½at², v² = u² + 2as - valid only for constant acceleration.
- Dynamics: ΣF = ma; friction f = μN; weight and normal force are not an action–reaction pair.
- Energy: the work–energy theorem applies generally; mechanical energy is conserved only when conservative forces alone do work.
- Fluids: P = P₀ + ρgh; continuity A₁v₁ = A₂v₂; buoyancy equals the weight of displaced fluid.
- Circuits: V = IR; resistances add in series, conductances add in parallel; P = VI = I²R.
- Waves and optics: v = fλ; n₁sinθ₁ = n₂sinθ₂; 1/f = 1/d₀ + 1/dᵢ with a consistent sign convention.
Worked example with reasoning
A car travelling at 20 m/s brakes uniformly at 5 m/s². How far does it travel before stopping? Taking the direction of motion as positive, a = −5 m/s² and final v = 0. From v² = u² + 2as: 0 = 400 + 2(−5)s, so 10s = 400 and s = 40 m. The common error is substituting a = +5, which returns a negative distance.
Traps that cost marks
- Sign conventions in braking and free fall.
- Mixing g = 9.8 with g = 10 inside one problem.
- Adding parallel resistances directly instead of using reciprocals.
- Leaving centimetres or grams in formulas that require SI units.
- Confusing image distance with focal length in lens questions.
Transfer tip
Before calculating, list the known quantities with units and the single unknown, then choose the equation containing only those symbols. This removes most dimensional errors.