1. The problem statement, all variables and given/known data This is a conceptual question about a prior assignment I had: A thin spherical shell rolls down an incline without slipping. If the linear acceleration of the center of mass of the shell is 0.23g, what is the angle the incline makes with the horizontal? 2. Relevant equations m*g*r*sin(theta) = Icm Alpha 3. The attempt at a solution This equation worked, but I was thinking about it after the fact, and I can't figure out why it worked. Torque = F*r, in this case the F is the force of gravity acting parallel to the incline, but that would only give me translational motion. The force causing torque comes from the friction between the plane and the wheel, which should be mg*u. So for this equation to work, there must be some other relationship between torque and translational motion. What I do know for a fact: 1. Linear acceleration is equal to angular acceleration multiplied by the radius. 2. The kinetic energies of rotation and translation are equal. So either there's another aspect of the relationship I'm missing, or the equation I used somehow accounts for the friction force. tl, dr: why does m*g*r*sin(theta) = Icm Alpha work when it should be m*g*u*r = Icm Alpha?