Physics - Mechanics, Electricity and Waves
The physics paper tests mechanics (kinematics, Newton's laws, work, energy, momentum), circular motion and gravitation, electricity and magnetism, waves and light, and introductory modern physics. Questions rarely ask for pure recall; they ask you to select the correct principle for a described situation, so identify the physics before touching the calculator.
Core rules
- Constant-acceleration kinematics: v = u + at, s = ut + ½at², v² = u² + 2as. These fail if acceleration is not constant.
- Newton's second law: ΣF = ma. Weight is W = mg and is a force, not a mass.
- Energy: KE = ½mv², gravitational PE = mgh; mechanical energy is conserved when only gravity does work.
- Momentum p = mv is conserved in collisions; impulse Ft = Δp.
- Ohm's law V = IR; series R = R₁ + R₂; parallel 1/R = 1/R₁ + 1/R₂; power P = VI = I²R.
- Waves: v = fλ; refractive index n = c/v.
Worked example
A 2 kg stone is released from rest at a height of 5 m. Find its speed just before impact, taking g = 9.8 m/s² and ignoring air resistance. Reasoning: only gravity does work, so mgh = ½mv². The mass cancels - a useful self-check - giving v = √(2gh) = √(2 × 9.8 × 5) = √98 ≈ 9.9 m/s. Verification by kinematics: v² = 0 + 2(9.8)(5) = 98, so v ≈ 9.9 m/s. Two routes, one answer: always check with the second method when time allows.
Traps that cost marks
- Confusing mass in kilograms with weight in newtons.
- Using g = 10 when the paper states 9.8, or dropping the negative sign for a body thrown upwards.
- Swapping the series and parallel resistance formulas.
- Failing to convert cm to m or g to kg before substitution.
- Assuming constant velocity in a situation where an acceleration is clearly stated.