pendulum

A fresco dating from 1841 in the Observatory Academy, Florence, Italy, which shows the 17-year-old Italian mathematician and astronomer Galileo contemplating a swinging lamp in Pisa Cathedral, and coming to the realization that each swing, long or short, takes the same time. It was only at the end of his life, nearly blind, that Galileo returned to the notion of a pendulum's regularity, and considered its application to clocks.

Weight (called a ‘bob’) swinging at the end of a rod or cord. The regularity of a pendulum's swing was used in making the first really accurate clocks in the 17th century. Pendulums can be used for measuring the acceleration due to gravity (an important constant in physics).

Specialized pendulums are used to measure velocities (ballistic pendulum) and to demonstrate the Earth's rotation (Foucault's pendulum).

For the use of a pendulum in parapsychology, see dowsing.