Why Is the Spherical Bob Preferred to Bobs of Other Shapes Used in a Simple Pendulum Experiment?
Technically, a pendulum can be created with an object of any weight or shape attached to the end of a rod or string. However, a spherical object is preferred because it can be most easily assumed that the center of mass is closest to the pivot's end, thereby limiting variables.
Simple equations that model the behavior of a pendulum are written in such a way to abstract the behavior of the pendulum in the most possible cases. Although in application, most pendulums do not have spherical bobs, the overall equation to describe a pendulum's movement can roughly predict the behavior anyway.