A bob is the weight on the end of a pendulum most commonly,
but not exclusively, found in pendulum clocks. Although a pendulum can
theoretically be any shape, any rigid object swinging on a pivot, clock
pendulums are usually made of a weight or bob attached to the bottom end of a
rod, with the top attached to a pivot so it can swing. The advantage of this
construction is that it positions the centre of mass close to the physical end
of the pendulum, farthest from the pivot. This maximizes the moment of inertia,
and minimizes the length of pendulum required for a given period. Shorter
pendulums allow the clock case to be made smaller, and also minimize the
pendulum's air resistance. Since most of the energy loss in clocks is due to
air friction of the pendulum, this allows clocks to run longer on a given power
source.
