A uniform disk with mass m and radius r is rotating with initial angular velocity oo, its axis passes through the center of this disk,how much time will it take before the disk is stopped (the kinetic friction coefficient is u) 0 solution dr =dF.r=udmg r m2x7办- 2mrdr R2 2mugr'dr dt= jd A uniform disk with mass m and radius r is rotating with initial angular velocity o， its axis passes through the center of this disk，how much time will it take before the disk is stopped（the kinetic friction coefficient is ） dr r o R solution： d  dF r   dmg r 2 2 2 2 R mrdr r dr R m dm       2 2 2 R m gr dr d    mgR R mgr dr d r     3 2 2 0 2 2     