1. The problem statement, all variables and given/known data Why is the attachment true? (It's from my notes) 2. Relevant equations General centre of mass equations 3. The attempt at a solution Really not sure how the lecturer did this. It's quoted as if it's obvious but I don't see it at all, this has been annoying me for a while. M(L-l) is obviously the moment of the ball about the center of masss.. I'm not sure where the other two terms come from really though.