drag

The force a fluid (air, water) exerts on an object moving through it, _opposing motion_. Two regimes matter for everyday physics: at low speed (or for small objects) drag is roughly proportional to velocity — `F_drag = k v`, called _linear_ or _Stokes_ drag. At high speed (or for large objects) it's roughly proportional to velocity squared — `F_drag = ½ ρ C_d A v²`, called _quadratic_ or _form_ drag. The proportionality constant absorbs cross-section, fluid density, and shape. Lemma's terminal-velocity page uses the linear case for clean closed-form math; raindrops and parachutists in real life sit in the quadratic regime, but the _shape_ of the answer (an asymptotic approach to a force-balance speed) is the same.