Pulling a Spool

A simple model for the kinetic frictional force on an object sliding across a surface is adopted in introductory physics. Its magnitude given by constant coefficient of friction multiplied normal exerted surface, and points opposite direction that slides relative to surface. In contrast, standard static more complicated. There no formula its magnitude, but instead it can vary from zero up some maximum value, as required keep point contact at rest on. Furthermore, way which would slip if there were friction, not always obvious is. attempt bypass these complications, students make erroneous assumptions about force. To counter them, helpful present example shows those need hold. Such system pulled spool rolls without slipping horizontal table. Since between table does slip, any them must be static.