The time rate of change of velocity. Since velocity is a directed or vector quantity involving both magnitude and direction, a velocity may change by a change of magnitude (speed) or by a change of direction or both. It follows that acceleration is also a directed, or vector, quantity. If the magnitude of the velocity of a body changes from υ₁ ft/s to υ₂ ft/s in t seconds, then the average acceleration a has a magnitude given by the following equation.