I'm stuck on a miniquestion in my HW that is hindering the entire problem, so here it is. A disc of mass M, radius R, Icm =1/2MR^2 is rolling down an incline dragging a mass M attached with a light rod to a bearing at the center of the disc. The friction coefficients are the same for both masses, us and uk. Determine the linear acceleration of the mass M. Well, this question seems easy, and I ended up with 2fs/M = a. But I need to find out what fs is to simplify my answer, and I can't figure out how.