The scripts and books that are used as guides for teaching Mechanics in the undergraduate laboratory of experimental physics in higher education often have an experiment that consists in dropping a metal ball on top of a curve launching platform suspended and attached on a support bench. Upon reaching the base of the platform, the ball is launching horizontally marking the reach range on a sheet using the carbon paper. This experience serves to illustrate the conservation of mechanical energy, assuming the hypothesis that the ball rolls without slipping, and the importance of taking into account its rotational kinetic energy. However, as the experiment is traditionally presented there is no description of the motion in real time so it is not possible to realize that the ball rolls and slides in most of the way, only reaching the condition of pure rolling at the end of the way. This misconception is the reason of the systematic difference between the predicted energy balance and that one obtained by experimental measurements. In this paper we develop a theoretical model that takes into account the curvature and the width of the track and using video-analysis we extract experimental data of the full movement that clearly illustrate a transition from rolling with slipping to pure rolling for different initial tilted angles analyzed, as the estimated value of the transition time for one of the cases.
video analysis; kinetic friction; energy conservation
