1. The problem statement, all variables and given/known data At the end of the lecture in http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/detail/embed13.htm [Broken], Prof. Walter Lewin compares the motion of a slider on a circular air track (no friction) with the motion of a ball on a circular path (with friction). Based on the first experiment, the period of the second turns out to be larger than expected. Prof. Lewin explains that the reason has nothing to do with friction, and leaves the question unsettled. 2. Relevant equations 3. The attempt at a solution I made an attempt at giving an explanation: in the second experiment, the ball may not be considered a particle, since it is rolling. I tried to derive the period of the oscillation taking this into account and got the following: T = 2pi * square root[ ((R-r)^2 + 2/5 R^2) / g(R-r) ], (using small angle approximation) where R is the radius of the track and r is the radius of the ball. If r << R, this is almost equal to T = 2pi * sqaure root[ 7/5 * R/g ] This exceeds the period of the slider on the air track by a factor of sqroot(7/5) = 1.183. Hence, the period may be expected to be larger by 18%. Is this correct? Thank you very much.