Physics
Core concepts
The physics section tests mechanics, energy, electricity and waves at secondary-school level. Know: SI units and prefixes; scalars versus vectors; speed, velocity and acceleration (v = u + at, s = ut + ½at², v² = u² + 2as); Newton's three laws and weight W = mg (g ≈ 9.8 m/s², often rounded to 10); work W = Fd, power P = W/t, kinetic energy ½mv², gravitational potential energy mgh; conservation of energy; pressure P = F/A and liquid pressure; density ρ = m/V; Ohm's law V = IR and series/parallel resistance; electrical power P = VI; basic wave ideas (frequency, wavelength, speed v = fλ); reflection and refraction.
Traps to expect
- Mass versus weight: mass in kg is constant; weight in newtons depends on g.
- Unit conversion: km/h to m/s requires dividing by 3.6; grams to kilograms requires dividing by 1000.
- Series versus parallel: in series, current is the same and voltages add; in parallel, voltage is the same and currents add. Resistance in parallel is always smaller than the smallest branch.
- Confusing energy and power: energy is joules, power is joules per second (watts).
- Free fall: ignoring that acceleration is downward and constant when air resistance is negligible.
Rule of thumb
Before substituting numbers, list the known quantities with units, convert them to SI, then choose the equation containing exactly one unknown. Carry units through the calculation to catch mistakes.
Worked example
A 2 kg object falls 5 m from rest. Using v² = u² + 2as with u = 0, a = 10 m/s², s = 5 m: v² = 100, so v = 10 m/s. Its kinetic energy is ½ × 2 × 10² = 100 J, matching the lost potential energy mgh = 2 × 10 × 5 = 100 J.