  Problems involving rotating systems analyzed from an inertial frame, without
invoking fictitious forces, is something that freshman students find difficult
to understand in an introductory mechanics course. One of the problems that I
workout in my intermediate mechanics class (which has students majoring in
physics as well as students majoring in engineering and other science
disciplines) is that of a bead sliding freely on a rod that is rotating
uniformly in a horizontal plane. The motivation for working out this problem
are two fold: (i) to train students in setting up Newton's equations of motion
by identifying force components and equating to the corresponding mass times
accelerations and (ii) to make the students familiar with the expressions for
acceleration in the polar coordinates that we introduce at the beginning of the
course. After guiding them through the solution which predicts an eventual
radially outward motion for the bead (except for a very special initial
condition), a typical question that gets asked is the following: The math is
all fine sir, but how does the bead that is initially at rest at some point on
the rod manage to move radially out if there are no radial forces? This article
is about the ways I have tried to answer this question over the years and help
students understand physics of the problem better.
