To analyze the problem, we consider the mechanical
energy of the system of the rod and the Earth. We choose
the configuration in which the rod is hanging straight down
as the reference configuration for gravitational potential
energy and assign a value of zero for this configuration.
When the rod is in the horizontal position, it has no rota-
tional kinetic energy. The potential energy of the system in
this configuration relative to the reference configuration is
MgL/2 because the center of mass of the rod is at a height
L/2 higher than its position in the reference configuration.
When the rod reaches its lowest position, the energy is en-
tirely rotational energy 1 2, where I is the moment of iner-
2
tia about the pivot, and the potential energy of the system is
zero. Because I 1 ML2 (see Table 10.2) and because the
3
system is isolated with no nonconservative forces acting, we
apply conservation of mechanical energy for the system: