  This paper aims to show how to guide students with a familiar example to
extract as much physics as possible before jumping into mathematical
calculation. The period for a physical pendulum made up of a uniform rod is
changed by attaching a piece of putty on it. The period for the combined system
depends on the location of the putty. Simple reasoning without calculation
shows that there are two locations for the putty that do not change the period
of the physical pendulum: the axis and the center of percussion. Moreover,
without calculation, we reason that there is at least one minimal period when
the putty lies somewhere between these two locations.

Classical Physics