A system of units is said to be gravitational if its units for length, time, and force are base units, that is, not defined in terms of other units. Any measurements in mechanics can be expressed in such a system. Gravitational systems are mainly used by engineers.
Gravitational systems contrast with absolute systems. In a gravitational system, the unit of force is a base unit, and the unit of mass is derived from it. In an absolute system, the unit of mass is a base unit, and the unit of force is derived from it.
The system is called “gravitational” because the unit of force is often defined by the effect of gravity on a physical prototype. For example, in the old British gravitational system, the unit for force was the pound force, defined as the force exerted by the prototype pound at a place where the acceleration of gravity is the standard 32.174 feet per second per second. The unit of mass, the slug, was then derived from the pound-force by defining it as that mass that will accelerate at 1 foot per second per second when a 1 pound-force acts upon it.
Gravitational systems are sometimes called technical systems.
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Last revised: 20 July 2004.