Physics
Motion
Separate speed (a scalar) from velocity (a vector). For constant acceleration use v = u + at and s = ut + ½at². A car starting from rest at 2 m/s² for 5 s reaches 10 m/s and travels 25 m.
Forces
Newton's second law is F = ma. Weight is mg; mass is not weight. Friction opposes relative motion. Action and reaction pairs act on different bodies, so they never cancel each other.
Energy and momentum
Work is force × distance; power is work ÷ time. Mechanical energy is conserved when friction is negligible. Momentum p = mv is conserved in collisions.
Waves and optics
v = fλ. Sound requires a medium; light does not. For mirrors and lenses, know which images are real and which are virtual, and remember that light bends toward the normal when entering a denser medium.
Electricity
Ohm's law: V = IR. In series, current is shared and resistances add; in parallel, voltage is shared and reciprocals of resistance add. Power P = VI.
Heat
Q = mcΔT for temperature change and Q = mL for a phase change.
Trap
Series and parallel rules are frequently swapped. Ask which quantity must be the same for every component before choosing a rule.