Drag
Definition
The resistance a fluid puts up against something moving through it — the reason you feel pushed back when you stick your hand out of a moving car window.
Symbol: · Unit: newtons (N)
Technical definition
The net fluid-dynamic force opposing relative motion between an object and a fluid, with components of skin-friction (viscous) drag and pressure (form) drag. At higher speeds it grows roughly with velocity squared: F_d = ½ρv²C_dA.
Example
A skydiver reaches terminal velocity — about 55 m/s belly-down — when air drag grows to exactly balance their weight. Tucking into a head-down position cuts drag and raises terminal velocity past 90 m/s.
Drag is not the same as friction, though both oppose motion — friction acts between solid surfaces in contact, while drag is the fluid's resistance. Also, drag never pushes you forward; even the best aerodynamics only reduces drag, never reverses it.