Kilogram-force

The unit kilogram-force (kgf, often just kg) or kilopond (kp) is defined as the force exerted by Earth's gravity on one kilogram of mass. Although the gravitational pull of the Earth varies as a function of position on earth, it is here defined as exactly 9.80665 m/s². So one kilogram-force is by definition equal to 9.80665 newtons.

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; 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.

It is also used for tension of bicycle spokes, for torque measured in "meter-kilograms", for pressure in kilograms per square centimeter, for the draw weight of bows in archery, and to define the "metric horsepower" (PS) as 75 m·kgf/s.

Grams-force and kilograms-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.

A tonne-force, metric ton-force, megagram-force, or megapond (Mp) is 1000 kilograms-force.

The decanewton (daN) is used in some fields as an approximation to the kilogram-force, being exactly rather than approximately 10 newtons.

External links

• BIPM SI brochure, chapter 2.2.2.