Angular acceleration is the rate of change of angular velocity. The average angular acceleration of an object whose angular velocity changes from ω1 to ω2 over time t is (ω2 - ω1)/t. Instantaneous acceleration is the value approached as t becomes small. The direction of the angular acceleration vector is perpendicular to the plane of motion. The tangential acceleration aT of a particle at a distance r from a fixed point is rα, where α is its angular acceleration.