The kilogram-force (kgf or kg
F), or kilopond (kp, from ), is a non-standard
gravitational metric unit of
force
In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
. It is not accepted for use 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 spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...
of
mass
Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
in a gravitational field (
standard gravity
The standard acceleration of gravity or standard acceleration of free fall, often called simply standard gravity and denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is a constant ...
, 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 a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition.
Some sta ...
of a kilogram under standard gravity. One kilogram-force is defined as .
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 s ...
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 .
History
The gram-force and kilogram-force were never well-defined units until the
CGPM
The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre ...
adopted a ''standard acceleration of gravity'' of 9.80665 m/s
2 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 a standard unit of force was explicitly rejected.
[Proceedings of the 3rd General Conference on Weights and Measures](_blank)
1901, pages 62–64 and 68, (french) Instead, the ''
newton'' was proposed in 1913
[Proceedings of the 5th General Conference on Weights and Measures](_blank)
1913, pages 51 and 56, (french) and accepted in 1948.
The kilogram-force has never been a part of the
International System of Units
The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...
(SI), which was introduced in 1960. The SI unit of force is the
newton.
Prior to this, the units were widely used in much of the world. They are still in use for some purposes; for example, they are used to specify 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 ...
s,
draw weight of bows in
archery
Archery is the sport, practice, or skill of using a Bow and arrow, bow to shooting, shoot arrows.Paterson ''Encyclopaedia of Archery'' p. 17 The word comes from the Latin ''arcus'', meaning bow. Historically, archery has been used for hunting ...
, and tensile strength of
electronics ''bond wire'', for informal references to pressure (as the technically incorrect ''kilogram per square centimetre'', omitting ''-force'', the kilogram''-force'' per square centimetre being the ''
technical atmosphere
Technical may refer to:
* Technical (vehicle), an improvised fighting vehicle
* Technical area, an area which a manager, other coaching personnel, and substitutes are allowed to occupy during a football match
* Technical advisor, a person who ad ...
'', the value of which is very near those of both the ''
bar'' and the ''standard atmosphere''), and to define the "
metric horsepower
Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are th ...
" (PS) as 75 metre-kiloponds per second.
In addition, the kilogram force was the standard unit used for
Vickers hardness test
The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell scale, Brinell method to measure the hardness of materials. The Vickers test is often easier to use than ot ...
ing.
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. Dividing the thrust in kilograms-force on the mass of an engine or a rocket in kilograms conveniently gives the
thrust to weight ratio
Thrust-to-weight ratio is a dimensionless quantity, dimensionless ratio of thrust to weight of a rocket, jet engine, Propeller (aircraft), propeller engine, or a vehicle propelled by such an engine that is an indicator of the performance of the eng ...
, dividing the thrust on propellant consumption rate (
mass flow rate
In physics and engineering, mass flow rate is the Temporal rate, rate at which mass of a substance changes over time. Its unit of measurement, unit is kilogram per second (kg/s) in SI units, and Slug (unit), slug per second or pound (mass), pou ...
) in kilograms per second gives the
specific impulse
Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine, such as a rocket engine, rocket using propellant or a jet engine using fuel, generates thrust. In general, this is a ratio of the ''Impulse (physics), ...
in seconds.
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]
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.
The gram-force is of a kilogram-force.
See also
*
Metrology
Metrology is the scientific study of measurement. It establishes a common understanding of Unit of measurement, units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to stan ...
*
Avoirdupois
Avoirdupois (; abbreviated avdp.) is a measurement system of weights that uses pounds and ounces as units. It was first commonly used in the 13th century AD and was updated in 1959.
In 1959, by international agreement, the definitions of the p ...
References
{{Reflist
Units of force
Non-SI metric units