Angular Acceleration

      When we switch on an electric fan, we notice that its angular velocity goes on increasing. We say that it has an angular acceleration ( change in velocity called acceleration ). Angular acceleration can be defined as "the rate of change of angular velocity". If ω_{i and} ω_{f are the values of instantaneous velocity of a rotating body at instants} t_{i and} t_{f, the average angular acceleration during the interval} t_f-t_{i is given by}

       The instantaneous angular acceleration is the limit of the ratio as approaches zero. Therefore, instantaneous angular acceleration is given by

      The angular acceleration is also a vector quantity whose magnitude is given by the above equation and whose direction is along the axis of rotation, given by right hand rule. Angular acceleration is expressed in units of /.