1. The problem statement, all variables and given/known data A uniform ring of mass m and radius r is projected horizontally with velocity v on a rough horizontal floor,so that it starts off with a pure sliding motion and it acquires a pure rolling motion after moving a distance d.If the coeffcient of friction between ground and ring is a,the work done by the friction in this process is, 2. Relevant equations w = fd 3. The attempt at a solution Since the ring starts with a pure sliding motion,the point of contact should move,since it is moving f =aN,N =mg so, f=amg and it begins to roll,after rolling motion starts,friction won't do any work so w=fd. so work done by friction is -amgd,but given answer is -0.5mv2 Please tell me where have I gone wrong.