1. The problem statement, all variables and given/known data A rigid ring is fixed with three bearings evenly spaced around its circunference, which allow it to rotate but not to displace in the radial direction. An external force is applied in a fixed point P of the ring which moves with the ring. The bearings have friction. These parameters from the problem are known: ring mass, radium and inertia momentum, external force (value F and angle phi) and friction coefficient. Obtain the expression for the angular acceleration as a function of these parameters and the angular position of the point P. 2. Relevant equations For the friction forces: F_fr = μN 3. The attempt at a solution I have tried projecting all the forces on the horizontal and vertical axis, and using Newtons Equations in the center of mass: ΣF = 0 and ΣM = Iα, but I end up with more unknown variables than equations.