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# Kilogram-force

kilogram-force
Unit systemGravitational metric system
Unit ofForce
Symbolkgf
Conversions
1 kgf in ...... is equal to ...
SI units   9.806650 N
CGS units   980,665.0 dyn
British Gravitational units   2.204623 lbf
Absolute English units   70.93164 pdl

The kilogram-force (kgf or kgF), or kilopond (kp, from Latin: pondus, lit.'weight'), is a non-standard gravitational metric unit of force. It is equal to the magnitude of the force exerted on one kilogram of mass in a 9.80665 m/s2 gravitational field (standard gravity, a conventional value approximating the average magnitude of gravity on Earth).[1] That is, it is the weight of a kilogram under standard gravity. Therefore, one kilogram-force is by definition equal to 9.80665 N.[2][3] Similarly, a gram-force is 9.80665 mN, and a milligram-force is 9.80665 μN.

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.[4]

## 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,[5] 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 explicitely rejected.[6] Instead, the newton was proposed in 1913[7] and accepted in 1948.[8] 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, for example, it is used for tension of bicycle spokes,[9] for informal references to pressure in kilograms per square centimetre (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.[2] In addition, the kilogram force was the standard unit used for Vickers hardness testing.

Three approaches to metric units of mass and force or weight[10][11]
Base Force Weight Mass
2nd law of motion m = F/a F = Wa/g F = ma
System GM M CGS MTS SI
Acceleration (a) m/s2 m/s2 Gal m/s2 m/s2
Mass (m) hyl kilogram gram tonne kilogram
Force (F),
weight (W)
kilopond kilopond dyne sthène newton
Pressure (p) technical atmosphere atmosphere barye pieze pascal

In 1940s, Germany, the thrust of a rocket engine was measured in kilograms-force,[citation needed] in the Soviet Union it remained the primary unit for thrust in the Russian space program until at least the late 1980s.[citation needed]

The term "kilopond" has been declared obsolete.[12]

## 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.

Units of force
newton
(SI unit)
dyne kilogram-force,
kilopond
pound-force poundal
1 N ≡ 1 kg⋅m/s2 = 105 dyn ≈ 0.10197 kp ≈ 0.22481 lbf ≈ 7.2330 pdl
1 dyn = 10–5 N ≡ 1 g⋅cm/s2 ≈ 1.0197 × 10–6 kp ≈ 2.2481 × 10–6 lbf ≈ 7.2330 × 10–5 pdl
1 kp = 9.80665 N = 980665 dyn gn ⋅ (1 kg) ≈ 2.2046 lbf ≈ 70.932 pdl
1 lbf ≈ 4.448222 N ≈ 444822 dyn ≈ 0.45359 kp gn ⋅ (1 lb) ≈ 32.174 pdl
1 pdl ≈ 0.138255 N ≈ 13825 dyn ≈ 0.014098 kp ≈ 0.031081 lbf ≡ 1 lb⋅ft/s2
The value of gn as used in the official definition of the kilogram-force is used here for all gravitational units.

## References

1. ^ The international system of units (SI)United States Department of Commerce, NIST Special Publication 330, 2008, p. 52
2. ^ a b NIST Guide for the Use of the International System of Units (SI) Special Publication 811, (1995) page 51
3. ^ BIPM SI brochure Archived 2004-06-15 at the Wayback Machine, chapter 2.2.2.
4. ^ NIST Guide to the SI, Chapter 5: Units Outside the SI
5. ^ Resolution of the 3rd CGPM (1901)
6. ^ Proceedings of the 3rd General Conference on Weights and Measures, 1901, pages 62–64 and 68, (french)
7. ^ Proceedings of the 5th General Conference on Weights and Measures, 1913, pages 51 and 56, (french)
8. ^ Resolution 7 of the 9th meeting of the CGPM (1948)
9. ^ Park Tool. "Balancing wheel tension with the TM-1 Spoke Tension Metre". Cyclingnews. Retrieved 2013-09-03. The recommended tension for spokes in bicycle wheels can be as low as 80 Kilograms force (Kfg) and as high as 230 Kilograms force.
10. ^ Comings, E. W. (1940). "English Engineering Units and Their Dimensions". Industrial & Engineering Chemistry. 32 (7): 984–987. doi:10.1021/ie50367a028.
11. ^ Klinkenberg, Adrian (1969). "The American Engineering System of Units and Its Dimensional Constant gc". Industrial & Engineering Chemistry. 61 (4): 53–59. doi:10.1021/ie50712a010.
12. ^ 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.