# angular acceleration

Angular acceleration is the rate of change of angular
velocity. The average angular acceleration of an object whose angular
velocity changes from *ω*_{1} to *ω*_{2} over time *t* is (*ω*_{2} - *ω*_{1})/*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 *a _{T}* of a particle at a distance

*r*from a fixed point is

*rα*, where

*α*is its angular acceleration.