A pendulum consists of only a few components including a length of string or wire, a bob or some type of weight and a fixed point. They can be used to prove that the planet rotates on an axis. The pendulum is a popular device used in watches and clocks.

A pendulum is traditionally defined as an object that hangs from one fixed point. When the object is set into motion, it is free to swing under the forces of gravity and inertia. A pendulum can be made from a length of cable or wire with some level of weight attached at one end; the other end is attached to a fixed point.

The longer the wire used in the creation of a pendulum, the longer it will take for the pendulum to complete one full swing or “period” after it has been set into motion. The weight of the bob generally has little effect on the motion of a pendulum. The point that a pendulum is fixed to must allow for fluid movement. To combat the force of air resistance from stopping a pendulum that has been set into motion, electromagnetic iron collars are used. They attract the wire and automatically turn on and off.

There are basically three forces that act on a pendulum when it is set in motion. These forces are inertia, gravity and air resistance. Inertia is the force that makes the pendulum swing outwards in a given direction. When a pendulum is set into motion, inertia keeps it moving. Gravity is the force that draws the pendulum back from the direction that inertia takes it in. Air resistance is the force that causes the pendulum to swing back and forth in shorter and shorter arcs. It is essentially the force that will eventually stop a pendulum from swinging.

Pendulums are used in time pieces because their consistent swinging can keep accurate time. The bobs on clock pendulums can be adjusted to make the clock run faster or slower. Sometimes, a clock may require multiple adjustments before it can be considered accurate.

The idea of the pendulum can be used to prove that Earth rotates on an axis. In 1851, Jean Bernard Leon Foucault demonstrated that the plane of oscillation on a 220-foot long pendulum that had been set into motion revolves about 270 degrees within a 24-hour period. This observation can prove that the Earth rotates on an axis.