1. The problem statement, all variables and given/known data A high-speed drill rotating ccw at 2200rpm comes to a halt in 2.80 s. What is the drill's angular acceleration? 2. Relevant equations (where w = angular velocity) w = 2╥/T v = wr a = v^2/r a = w^2*r 3. The attempt at a solution I start by calculating w. So first I took it to rev per second and then second per revolution which came to be 0.027 seconds per revolution. So w = 2╥/0.027 = 232.71 radians. Now I'm stuck because I don't have r (radius). I've tried going backwards to get r from v, but to no success since v depends in r as r depends in v. Every equation relating to acceleration seems to need r but I don't see how I can get it. Any ideas would be appreciated. Thanks.