# pound-force

A pound or pound-force (abbreviations: lb, lbf, or lbf) is a unit of force. Pound is also the name of a unit of mass. One pound-force is approximately equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth.

The standard acceleration due to Earth's gravity is usually taken to be 9.80665 m/sÂ² (approximately 32.174 05 ft/sÂ²) today,[1] but other values have been used, including 32.16 ft/sÂ² (approximately 9.80237 m/sÂ²).[2] The actual acceleration due to Earth's gravity varies from place to place, in general increasing from the equator (9.78 m/sÂ²) to the poles (9.83 m/sÂ²).

## Equivalence to other units of force

A pound-force is the gravitational force exerted on a pound-mass in the standard gravitational field at Earth's surface which causes free falling bodies to accelerate at exactly 9.80665 m/sÂ² (32.1742 ft/sÂ²) (see relationships table). An international avoirdupois pound is exactly 453.59237 grams or 0.45359237 kg. This means that 1 pound-force is equal to (0.45359237 Ã— 9.80665) newtons, or approximately 4.448222 newtons (conversely, 1 newton is 0.22481 pounds-force).
 1 N 1 dyn 1 kp 1 lbf Newton(SI unit) Dyne Kilogram-force(Kilopond) Pound-force Poundal ≡ 1 kgm/s = 105 dyn ≈ 0.10197 kp ≈ 0.22481 lbf ≈ 7.2330 pdl = 10−5 N ≡ 1 gcm/s ≈ 1.019710−6 kp ≈ 2.248110−6 lbf ≈ 7.233010−5 pdl = 9.80665 N = 980665 dyn ≡ gn(1 kg) ≈ 2.2046 lbf ≈ 70.932 pdl ≈ 4.448222 N ≈ 444822 dyn ≈ 0.45359 kp ≡ gn(1 lb) ≈ 32.174 pdl ≈ 0.138255 N ≈ 13825 dyn ≈ 0.014098 kp ≈ 0.031081 lbf ≡ 1 lbft/s The value of gn as used in the official definition of the kilogram-force is used here for all gravitational units.

The pound-force has the same relationship to the 'ounce' used as a unit of force as the pound (unit of mass) has to the ounce (unit of mass).

## Use of the pound as a unit of force

In some contexts, the unit "pound" refers to a unit of mass (see Use in Commerce under Pound (mass)). However, in other contexts, by convention, the "pound" refers to a unit of force. In circumstances where there may be ambiguity otherwise, the symbol "lbf" or the term "pounds-force" can be used for the unit of force and the term "pounds-mass" ("lbm") can be used for the unit of mass.

For example, in structural engineering applications the term "pound" is used almost exclusively to refer to a unit of force and not to refer to the unit of mass. In those applications, the preferred unit of mass is the slug, i.e. lbfÂ·sÂ²/ft.

There are three practical ways of doing calculations with mass and force in the foot-pound-second (fps) systems (and other systems such as inch-pound-second systems not discussed here). Those three ways are summarized in the table below, which also sets out the corresponding position under the International System of Units (SI).

Force-mass relationships
Systems Gravitational Engineering Absolute
Newton’s second law F = mÂ·aF = mÂ·a/gc = wÂ·a/gF = mÂ·a
Weight of an object w = mÂ·gw = mÂ·g/gcw = mÂ·g
Units EnglishMetricEnglishMetricEnglishMetric
Timesecondsecondsecondsecondsecondsecond
Distancefootmeterfootmeterfootmeter
Massslughylpound-masskilogrampoundkilogram
Forcepoundkilopondpound-forcekilopondpoundalnewton

The SI system and the absolute and gravitational fps systems are coherent systems of units. They have the advantage that force can be expressed as the product of mass and acceleration.

The "engineering" fps system requires the introduction of a gravitational constant, gc, into the relationship between force on the one hand and mass and acceleration on the other hand. In this system, the weight of the mass unit (pound [mass]) on Earth's surface is equal to the force unit (pound-force). The price for this convenience is that the force unit is no longer equal to the mass unit multiplied by the distance unit divided by the time unit squared (the use of Newton's Second Law, F = ma, requires another factor, gc). This gravitational constant is usually taken to be 32.17405 lbÂ·ft/(lbfÂ·sÂ²). The required introduction of the constant gc is a reason why many, including people in engineering fields, prefer the simpler "gravitational" fps system, and use the slug as the unit of mass.

No one of the three fps systems is more correct than the other two.

## Historical origins

Pounds-force had been used in low-precision measurements since the 18th century, but they were never well-defined units until the 20th century.

The second resolution of the third General Conference on Weights and Measures (CGPM) in 1901 declared that:[3] The value adopted in the International Service of Weights and Measures for the standard acceleration due to Earth's gravity is 980.665 cm/sÂ², value already stated in the laws of some countries. This value was the conventional reference for calculating the kilogram-force, a unit of force whose use has been deprecated since the introduction of the SI.[1]

## References

1. ^ Barry N. Taylor, Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811, Appendix B note 24[2]
2. ^ J. Edmond Shrader, Physics for Students of Applied Science, McGraw Hill, 1st ed., 1937, p. 24.
3. ^ Resolution of the 3rd CGPM
units of measurement have played a crucial role in human endeavour from early ages up to this day. Disparate systems of measurement used to be very common. Now there is a global standard, the International System (SI) of units, the modern form of the metric system.
In physics, force is an action or agency that causes a body of mass m to accelerate. It may be experienced as a lift, a push, or a pull. The acceleration of the body is proportional to the vector sum of all forces acting on it (known as net force or resultant force).
pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United
pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United
EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001. Their greatest hit, their debut single "time after time", peaked at #13 in the Oricon singles chart.
Standard gravity, usually denoted by g0 or gn, is the nominal acceleration due to gravity at the Earth's surface at sea level. By definition it is equal to exactly 9.80665  mÂ·s−2 (approx. 32.174 ftÂ·s−2).
Earth's gravity, denoted by g, refers to the attractive force that the Earth exerts on objects on or near its surface (or, more generally, objects anywhere in the Earth's vicinity).
The newton (symbol: N) is the SI derived unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics.

## Definition

A newton
The newton (symbol: N) is the SI derived unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics.

## Definition

A newton
Si, si, or SI may refer to (all SI unless otherwise stated):

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Dynes redirects here. For the president of the University of California system, see Robert C. Dynes.

In physics, the dyne (symbol "dyn") is a unit of force specified in the centimeter-gram-second
The unit kilogram-force (kgf, often just kg) or kilopond (kp) is defined as the force exerted by one kilogram of mass in standard Earth gravity.
The poundal is a non-SI unit of force. It is a part of the foot-pound-second system of units, a coherent subsystem of English units introduced in 1879, and one of several specialized subsystems of mechanical units used as aids in calculations.
g-force (also g-load) is a measurement of an object's acceleration expressed in g's. It may also informally refer to the reaction force resulting from an acceleration, with the causing acceleration expressed in g's.
pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United
1 foot =
SI units
0 m 0 mm
US customary / Imperial units
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A foot (plural: feet or foot;[1] symbol or abbreviation: ft or, sometimes,
pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United
ounce (abbreviation: oz) is the name of a unit of mass in a number of different systems, including various systems of mass that form part of English units, Imperial units, and United States customary units. Its size can vary from system to system.
pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United
pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United
The slug is an English unit of mass. It is a mass that accelerates by 1 ft/sÂ² when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of about 32.17405 pound-mass or 14.5939 kg.
International System of Units (abbreviated SI from the French Le SystÃ¨me international d'unitÃ©s) is the modern form of the metric system.
The slug is an English unit of mass. It is a mass that accelerates by 1 ft/sÂ² when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of about 32.17405 pound-mass or 14.5939 kg.
The slug is an English unit of mass. It is a mass that accelerates by 1 ft/sÂ² when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of about 32.17405 pound-mass or 14.5939 kg.
The unit kilogram-force (kgf, often just kg) or kilopond (kp) is defined as the force exerted by one kilogram of mass in standard Earth gravity.