The kilogram-force (kgf or kgF), or kilopond (kp, from Latin pondus meaning weight), is a gravitational metric unit of force. It is equal to the magnitude of the force exerted by one kilogram of mass in a 7000980665000000000♠9.80665 m/s2 gravitational field (standard gravity, a conventional value approximating the average magnitude of gravity on Earth). Therefore one kilogram-force is by definition equal to 7000980665000000000♠9.80665 N. Similarly, a gram-force is 6997980665000000000♠9.80665 mN, and a milligram-force is 6994980664999999999♠9.80665 µN. One kilogram-force is approximately 2.204622 pounds-force.
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Kilogram-force is a non-standard unit and does not comply with the SI Metric System.
History
The gram-force and kilogram-force were never well-defined units until the CGPM adopted a standard acceleration of gravity of 980.665 cm/s2 for this purpose in 1901, though they had been used in low-precision measurements of force before that time. The kilogram-force has never been a part of the International System of Units (SI), which was introduced in 1960. The SI unit of force is the newton.
Prior to this, the unit was widely used in much of the world and it is still in use for some purposes. The thrust of a rocket engine, for example, was measured in kilograms-force in 1940s Germany, in the Soviet Union (where it remained the primary unit for thrust in the Russian space program until at least the late 1980s), and it is still used today in China and sometimes by the European Space Agency.
The term "kilopond" has been declared obsolete and should no longer be used.
It is also used for tension of bicycle spokes, for informal references to pressure in kilograms per square centimeter (1 kp/cm2) which is the technical atmosphere (at) and very close to 1 bar and the standard atmosphere (atm), for the draw weight of bows in archery, and to define the "metric horsepower" (PS) as 75 metre-kiloponds per second. In addition, kilograms force is the standard unit used for Vickers hardness testing.
Related units
The tonne-force, metric ton-force, megagram-force, and megapond (Mp) are each 1000 kilograms-force.
The decanewton or dekanewton (daN) is used in some fields as an approximation to the kilogram-force, being exactly rather than approximately 10 newtons.