Physics: Principles, Units, and Structured Reasoning
Core concepts
Physics rewards understanding relationships, not memorising formulas. Cover mechanics (Newton's laws, momentum, work-energy, circular motion), gravitation, waves and optics, heat and thermodynamics, electrostatics, current electricity, magnetism, and modern physics. Be fluent with SI units, dimensional analysis, and significant figures. Know when conservation laws (energy, momentum, charge) give a shortcut.
Traps in questions
- Direction vs magnitude: force, velocity, and momentum are vectors; omitting direction loses marks.
- Unit conversion: cm to m, g to kg, and minutes to seconds before substitution.
- Ideal assumptions: massless strings, frictionless surfaces, and ideal gases are stated conditions, not optional.
- Graph interpretation: the gradient and area under a graph have physical meaning (e.g. area under a velocity–time graph is displacement).
- Formula recall without conditions: $v = u + at$ only applies for constant acceleration.
Worked example
A 2 kg object accelerates uniformly from rest to 10 m/s in 4 s. Find the net force. First find acceleration: $a = \frac{v-u}{t} = \frac{10-0}{4} = 2.5\ \text{m/s}^2$. Then $F = ma = 2 \times 2.5 = 5\ \text{N}$ in the direction of motion. Check with work-energy: $W = \Delta KE = \tfrac12(2)(100) = 100\ \text{J}$, and $W = Fs$ with $s = \tfrac12 at^2 = 20\ \text{m}$, giving $F = 5\ \text{N}$. Consistent.
Rule of thumb
Write knowns with units, state the principle you are using, then substitute. A labelled answer with units and direction earns method marks even if arithmetic slips.