Hello, I would like to be sure about my rotationnal velocities w1(t) and w2(t): http://imagizer.imageshack.us/v2/800x600q90/912/c6X2du.png [Broken] The black arm is turning clockwise at w1. The blue disk is turning at w2 with w2 < w1. w1 and w2 are in labo frame reference. The disk is turning around itself counterclockwise at w2'=w2-w1 in the arm frame reference. I1: inertia of black arm+disk I2: inertia of the disk F: force from friction r: radius of the disk t: time. I guess no friction elsewhere than between disk and black arm. I guess that the force F from friction is constant even rotationnal velocities are changing, just for simplify calculations. Friction gives forces F1 and F2. For me, while w2<w1 and while the force F can be constant : w1(t)=w1-Frt/I1 w2(t)=w2+Frt/I2-Frt/I1 For you, is it ok ?