The force that opposes an object’s motion through a fluid is drag.
Drag, also called fluid resistance, acts opposite to an object’s motion relative to a surrounding fluid. Air and water are both fluids, so drag affects aircraft, cars, boats, swimmers, falling objects, and even particles moving through liquids or gases. The force arises from pressure differences around the object and from friction-like effects caused by the fluid’s viscosity.
For many ordinary objects moving through air, drag is described by the equation Fᴅ = ½ρv²CᴅA. Here, ρ is fluid density, v is relative speed, Cᴅ is the drag coefficient, and A is the reference area. Because speed is squared, doubling speed can produce roughly four times as much drag when the other factors remain constant.
Drag is not identical to ordinary solid-surface friction, although skin-friction drag is one component of aerodynamic drag. It also differs from lift, which acts perpendicular to the flow, while drag acts parallel and opposite to motion. At very low speeds, drag can be proportional to speed rather than speed squared, depending on the flow regime and Reynolds number.