In most physical systems, the effects of friction are
not easily predicted, or even measured. The interactions
between objects that cause them to resist sliding
against each other seem to be due in part to microscopic
irregularities of the surfaces, but also in part to
interactions on a molecular level. However, though
the phenomena is not fully understood, there are
some properties of friction that hold for most materials
under many different conditions. In this experiment
you will investigate some of the properties of
sliding friction—the force that resists the sliding
motion of two objects when they are already in
motion.