Physics
Scope
Motion and kinematics, Newton's laws, momentum and collisions, work, energy and power, circular motion and gravitation, waves and sound, light and optics, heat and thermodynamics, electricity and magnetism, and light modern-physics topics.
Method over memory
Most numerical items need only three or four equations: v = u + at, s = ut + ½at², v² = u² + 2as, F = ma, W = Fd, P = W/t, and for electricity V = IR with series and parallel rules. Write the known quantities with units, choose the equation containing exactly one unknown, substitute, then check the units of the answer.
Traps
Mixing units (cm with m, grams with kilograms) before substituting. Dropping the sign of acceleration when an object is decelerating or thrown upward. Using g = 10 or 9.8 inconsistently with the given options. Confusing mass and weight. Adding resistances in parallel as if they were in series. Confusing average speed (total distance ÷ total time) with average velocity (displacement ÷ time). In optics, forgetting that light bends towards the normal when entering a denser medium.
Worked example
Two resistors of 6 Ω and 3 Ω are connected in parallel. 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so R = 2 Ω. In series the same pair would give 9 Ω - read the circuit description carefully before calculating.
Rule of thumb
Convert every quantity to SI units first, and if the answer looks wrong by a factor of 10 or 100, suspect a unit conversion rather than a wrong formula.