I.

Introduction

Pendulum, device consisting of an object suspended from a fixed point that swings back and forth under the influence of gravity (see Gravitation). Pendulums are used in several kinds of mechanical devices; for example, certain types of clocks use pendulums (see Clocks and Watches).

The most basic type of pendulum is the simple pendulum. In a simple pendulum, which oscillates back and forth in a single plane, all the mass of the device can be considered to reside entirely in the suspended object. The motion of pendulums such as those in clocks closely approximates the motion of a simple pendulum. A spherical pendulum is not confined to a single plane, and as a result its motion can be much more complicated than the motion of a simple pendulum.

The principle of the pendulum was discovered by Italian physicist and astronomer Galileo, who established that the period for the back-and-forth oscillation of a pendulum of a given length remains the same, no matter how large its arc, or amplitude. (If the amplitude is too large, however, the period of the pendulum is dependent on the amplitude.) This phenomenon is called isochronism, and Galileo noted its possible applications in timekeeping. Because of the role played by gravity, however, the period of a pendulum is related to geographical location, because the strength of gravity varies as a function of latitude and elevation. For example, the period will be greater on a mountain than at sea level. Thus, the pendulum can be used to determine accurately the local acceleration of gravity.