Determination of skin friction on a rotating sphere in magnetic levitation

Abstract

A simple and inexpensive experimental procedure is proposed to investigate the skin friction caused by air in a rotating sphere. A device using a magnetic field keeps the sphere levitating. The sphere is briefly boosted and begins to rotate, but the air resistance acts in opposition to the motion and consequently the period of rotation increases. The experimental data demonstrate that the rotation period of the sphere increases exponentially with time. To account for this behavior, a physical model was developed, which provides an accurate description of the data. A coefficient, similar to the drag coefficient in the Drag Equation, was used to measure the air resistance on the rotating sphere. It is denominated as skin friction coefficient, its value was determined and it is remarkable that such a simple experiment provided a result so close to similar parameter value found in the literature.