1. The problem statement, all variables and given/known data A solid sphere of mass m and radius a can rotate freely about a point A on its surface. The sphere is held initially at rest with the line OA through A and the centre of the sphere O horizontal and is released under gravity. Find the angular velocity of the system when OA first becomes vertical. (You may assume that the moment of inertia of the sphere about an axis through O is (2/5)ma^2.) 2. Relevant equations energy change .5*Iz*w^2 = mgh where Iz is the moment of inertia about z w is angular velocity mgh potential energy 3. The attempt at a solution parallel axis theorem Iz = (2/5)ma^2 + ma^2 = (7/5)ma^2 energy change .5*Iz*w^2 = mgh (where h is the change in height of centre of mass) (7/10)*m*a^2*w^2 = mga rearranging w = (100/(7a))^.5 I'm not sure how to find a or if I even have enough information to solve for a. Should I try angular momentum?