The rate at which one system changes its position with respect to another system. When a body is moving in a fluid, in a direction opposite to the flow of the fluid, the magnitude of the body's relative velocity is the algebraic sum of the magnitude of its own velocity and that of the fluid. The relative velocity of the body will affect the magnitude of the forces (e.g. drag and lift) exerted by the fluid on the body.
Subjects: Sports and Exercise Medicine.