Pendulum, an object connected to a fixed support in such a way that it is free to swing back and forth under the influence of gravity. The typical pendulum consists of a weight, or bob, and a wire or slender rod by which the bob is suspended. At rest, the bob hangs directly below the point of support; if the bob is moved from this position and released, gravity causes it to swing back and forth along a circular arc at the end of the wire or rod. When the bob reaches the end of one swing, gravity causes it to begin to fall and gain speed as it moves downward. After it passes the lowest point in the swing, the bob begins to move upward along the arc. Gravity then causes the bob to slow down until it stops and begins to fall again. The bob continues to swing back and forth in the same vertical plane unless acted upon by an outside force.

The time it takes for a pendulum to oscillate from the peak of a swing on one side to the other side and back is called the period of vibration. In general, the period depends on the length of the pendulum, the magnitude of gravitational acceleration where the pendulum is located, and the amplitude of the swing. If the amplitude is small, it has virtually no effect on the period. In that case, the period is given by the equation T=2p l/g where T is the period of vibration, l is the length of the pendulum, and g is the local gravitational acceleration. The Greek letter ∏ (pi) is a constant with an approximate value of 3.1416.

Thus when the amplitude of the swing is small, the period of vibration of a pendulum is directly proportional to the square root of its length. For example, if a pendulum one meter long has a period of one second, a pendulum four meters long will have a period of two seconds.

Because of the uniformity of its period, a pendulum can be used in keeping time. Pendulums can also be used to determine gravitational acceleration at a particular location.

In 1851, Jean Bernard Leon Foucault used a long pendulum to make the first conclusive demonstration of the earth's rotation. The pendulum was free to swing in any direction with respect to its support and had a bob that traced a line in loose sand on each swing. After the pendulum was set in motion, the tracings showed that the plane of the pendulum's swing seemed to turn slowly in a clockwise direction. Since the vertical plane in which a pendulum swings does not change once the pendulum is set in motion, the tracings indicated that the earth must be turning. Pendulums that demonstrate the rotation of the earth are known as Foucault pendulums; they are exhibited in many museums.