Physics
What the paper tests
The Physics section tests mechanics, gravitation, rotational motion, simple harmonic motion, waves and sound, heat and thermodynamics, electrostatics, current electricity, magnetism, optics and a slice of modern physics. Questions are numerical or conceptual, and the conceptual ones usually hinge on a sign convention, a direction, or a limiting case rather than a formula you have not seen.
Core rules to have at your fingertips
- Equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as, valid only for uniform acceleration.
- Projectile: T = 2u sinθ/g, H = u²sin²θ/2g, R = u²sin2θ/g, with horizontal velocity constant.
- Work–energy theorem: W_net = ΔKE; for a spring, U = ½kx².
- Lens/mirror: 1/f = 1/v − 1/u for lenses (Cartesian sign convention); magnification m = v/u.
- Ohm's law with internal resistance: I = E/(R + r), terminal p.d. V = E − Ir.
- Thermodynamics: ΔQ = ΔU + ΔW; efficiency of a Carnot engine = 1 − T₂/T₁ in kelvin.
Worked example
A stone is thrown at 20 m/s at 30° above the horizontal from level ground. Find its range using g = 10 m/s². Resolve: uₓ = 20 cos30° = 17.32 m/s; u_y = 20 sin30° = 10 m/s. Time of flight: T = 2u_y/g = 2(10)/10 = 2 s. Range = uₓ × T = 17.32 × 2 = 34.6 m. Cross-check with R = u²sin2θ/g = (400)(sin60°)/10 = 40 × 0.866 = 34.6 m. Agreement confirms the resolution step.
Traps that cost marks
- Using g = 9.8 in one line of a solution and 10 in another; adopt the value the options imply.
- Forgetting that at maximum height the vertical velocity is zero but the horizontal velocity is unchanged.
- Sign errors in the lens formula, especially for virtual images and concave surfaces.
- In circuits, adding internal resistance to the external resistance but then applying V = IR across the terminals.
- Carnot efficiency computed from Celsius temperatures.