A well-known problem in introductory mechanics concerns a ball that rolls or slides down the surface of a sphere.1–3 Simple analytical solutions can be obtained if it is assumed that energy is conserved,2 but the problem is more complicated if sliding friction is involved.3 Two experimental results have been reported, one that is very simple and another that is more sophisticated. The simple method involves coating a metal ball with finger paint and observing the track on the sphere to determine where the ball loses contact.2 Given that the normal reaction force decreases to zero at that point, and that the ball plus the paint might be slippery, it is not certain that the track ends exactly where the ball loses contact or that energy is conserved.
The sophisticated method3 uses equipment that is not generally available in a high school laboratory, and also uses...
