1. The problem statement, all variables and given/known data I have a conceptual question which is not HW, although the moderators of this forum think different. In the "hoop" example, and the example after that here: http://tonic.physics.sunysb.edu/~dteaney/F06_Ph2034/lectures/lexam3.pdf do you ALWAYS have the change in gravitational potential energy as the difference in the height of the CENTER OF MASS? Rewording: When you use ##mgh + 1/2mv^2 + 1/2 I\omega^2 = (mgh + 1/2mv^2+1/2I\omega^2 )f##, is Mgh ALWAYS the difference in height of the CENTER OF MASS? or can it [the center of mass] be any point on the object? 2. Relevant equations see above 3. The attempt at a solution This is not a homework problem. Thanks, and hope this is answered before AP tests!