Pendulum

A gravity pendulum (plural pendula) is a weight on the end of a rigid rod (or a string/rope), which, when given an initial push, will swing back and forth under the influence of gravity over its central (lowest) point. A torsion pendulum consists of a body suspended by a fine wire or elastic fiber in such a way that it executes rotational oscillations as the suspending wire or fiber twists and untwists. Another variety of a torsion pendulum is a fixed elastic coil connected to a rod-like object; once moved off its resting position, the coil will set the rod into an oscillatory motion.
Galileo Galilei discovered that pendula exhibit regular periodic motion, a feature which he correctly speculated would make them useful for timekeeping devices such as clocks.