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The kilogram-force (kgf or kgF), or kilopond (kp, from la, pondus, lit=weight), is a non-standard gravitational metric unit of
force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...
. It does not comply with the International System of Units (SI) and is deprecated for most uses. The kilogram-force is equal to the magnitude of the force exerted on one
kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...
of
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...
in a gravitational field (
standard gravity The standard acceleration due to gravity (or standard acceleration of free fall), sometimes abbreviated as standard gravity, usually denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth ...
, a conventional value approximating the average magnitude of gravity on Earth). That is, it is the
weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar q ...
of a kilogram under standard gravity. Therefore, one kilogram-force is by definition equal to .
NIST The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sc ...
br>''Guide for the Use of the International System of Units (SI)''
Special Publication 811, (1995) page 51 Similarly, a gram-force is , and a milligram-force is . Kilogram-force is a non-standard unit and is classified in the International System of Units (SI) as a unit that is not accepted for use with SI.


History

The gram-force and kilogram-force were never well-defined units until the CGPM adopted a ''standard acceleration of gravity'' of 9.80665 m/s2 for this purpose in 1901, though they had been used in low-precision measurements of force before that time. Even then, the proposal to define kilogram-force as standard unit of force was explicitly rejected.Proceedings of the 3rd General Conference on Weights and Measures
1901, pages 62–64 and 68, (french) Instead, the '' newton'' was proposed in 1913Proceedings of the 5th General Conference on Weights and Measures
1913, pages 51 and 56, (french) and accepted in 1948.Resolution 7 of the 9th meeting of the CGPM (1948)
/ref> 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, for example, it is used for the tension of bicycle
spoke A spoke is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface. The term originally referred to portions of a log that had been riven (split l ...
s, for informal references to pressure in kilograms per square centimetre (1 kp/cm2) which is the
technical atmosphere Technical may refer to: * Technical (vehicle), an improvised fighting vehicle * Technical analysis, a discipline for forecasting the future direction of prices through the study of past market data * Technical drawing, showing how something is c ...
(at) and very close to 1
bar Bar or BAR may refer to: Food and drink * Bar (establishment), selling alcoholic beverages * Candy bar * Chocolate bar Science and technology * Bar (river morphology), a deposit of sediment * Bar (tropical cyclone), a layer of cloud * Bar (un ...
and the standard atmosphere (atm), for the draw weight of bows in archery, for the strength of bond wire in grams-force, and to define the " metric horsepower" (PS) as 75 metre-kiloponds per second. In addition, the kilogram force was the standard unit used for Vickers hardness testing. In 1940s, Germany, the thrust of a rocket engine was measured in kilograms-force, in the Soviet Union it remained the primary unit for thrust in the Russian space program until at least the late 1980s. The term "kilopond" has been declared obsolete.European Economic Community, Council Directive of 18 October 1971 on the approximation of the laws of the Member States relating to units of measurement (Directive 71/354/EEC), Annex, Chapter III.


Related units

The tonne-force, metric ton-force, megagram-force, and megapond (Mp) are each 1000 kilograms-force. The decanewton or dekanewton (daN), exactly 10 N, is used in some fields as an approximation to the kilogram-force, because it is close to the 9.80665 N of 1 kgf.


See also

*
Metrology Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fra ...
* Avoirdupois


References

{{Reflist Units of force Non-SI metric units