A pendulum is a mechanical device consisting of a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting mechanical equilibrium position, it is subject to a restoring force due to gravity, which will accelerate it back toward the equilibrium position. When released, the restoring force acting on the pendulum's mass causes it to oscillate about the equilibrium position, swinging back and forth. The time for one complete cycle, a left swing and a right swing, or frequency, is commonly known as its "period". The period depends both on the length of the pendulum and, somewhat, on the amplitude. The SI unit of the period of a pendulum is the second (s).