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foot-pound-second system of units (fps system of units)

The foot-pound-second (fps) system of units is a scheme for measuring dimensional and material quantities. The fundamental units are the foot for length, the pound for weight, and the second for time.

The fps system has two variants, known as the American version and the Imperial version. Neither scheme is often used by scientists nowadays; the International System of Units (SI) is preferred. However, fps units are used to some extent by the general public, especially in the United States.

One foot (1 ft) represents a length of 12 inches. The inch was originally defined as the length of three typical barleycorns laid end-to-end. A foot was also approximately equal to three hand widths or 2/3 of a cubit (the distance from an average person's elbow to the tips of the fingers). Nowadays, a foot is considered to be 0.3048 meter, where the meter is the fundamental unit of displacement in the metric system and International System of Units (SI).

One pound (1 lb) is the force that produces an acceleration of 32.1740 feet per second squared (32.1740 ft/sec2) when applied against a known standard mass. The acceleration of 32.1740 ft/sec2 is approximately the value of the earth's gravitational acceleration at 45 degrees north latitude.

One second (1 s or 1 sec) is the time that elapses during 9.192631770 x 109 cycles of the radiation produced by the transition between two levels of Cesium 133. It is also 1/86,400 of a mean solar day. (There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day; 60 x 60 x 24 = 86,400.)

This was last updated in March 2011

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