In
fluid dynamics,
drag (sometimes called
air resistance, a type of friction, or
fluid resistance, another type of
friction or fluid friction) refers to
forces acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers (or surfaces) or a fluid and a
solid surface. Unlike other resistive forces, such as dry
friction, which are nearly independent of velocity, drag forces depend on velocity. Drag force is proportional to the velocity for a
laminar flow and the squared velocity for a
turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of
viscosity.