The pound or pound force (symbol: lb, lb_{f}, lbf) is a unit of force in some systems of measurement including English engineering units and British gravitational units.^{[1]}
Definitions
The poundforce is equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth. Since the 18th century, the unit has been used in lowprecision measurements, for which small changes in Earth's gravity (which varies from place to place by up to half a percent) can safely be neglected.^{[2]}
The 20th century, however, brought the need for a more precise definition. A standardized value for acceleration due to gravity was therefore needed. Today, in accordance with the General Conference on Weights and Measures, standard gravity is usually taken to be 9.80665 m/s^{2} (32.174 049 ft/s^{2}).^{[3]}^{[4]}
The acceleration of the standard gravitational field (g_{n}) and the international avoirdupois pound (lbm) define the poundforce as:^{[5]}
 $\backslash begin\{align\}$
1\,\mathrm{lbf} &= 1\,\mathrm{lbm} \times g_{\rm n} \\
&= 1\,\mathrm{lbm} \times 32.174049\,\mathrm{\tfrac{ft}{s^2}}\\
&= 32.174049\,\mathrm{\tfrac{ft {\cdot} lbm}{s^2}}\end{align}
 $\backslash begin\{align\}$
or\end{align}
 $\backslash begin\{align\}$
1\,\mathrm{lbf} &= 0.45359237\,\mathrm{kg} \times 9.80665\,\mathrm{\tfrac{m}{s^2}}\\
&= 4.4482216152605\,\mathrm{N} \text{ (exact)}\end{align}
This definition can be rephrased in terms of the slug. A slug has a mass of 32.174049 lbm. A poundforce is the amount of force required to accelerate a slug at a rate of 1 ft/s^{2}, so:
 $1\backslash ,\backslash mathrm\{lbf\}\; =\; 1\backslash ,\backslash mathrm\{slug\}\; \backslash times\; 1\backslash ,\backslash mathrm\{\backslash tfrac\{ft\}\{s^2\}\}$
 $=\; 1\backslash ,\backslash mathrm\{slug\}\backslash ,\backslash mathrm\{\backslash tfrac\{ft\}\{s^2\}\}$
Conversion to other units
Units of force

newton (SI unit) 
dyne 
kilogramforce, kilopond 
poundforce 
poundal

1 N

≡ 1 kg·m/s^{2}

= 10^{5} dyn

≈ 0.10197 kp

≈ 0.22481 lb_{F}

≈ 7.2330 pdl

1 dyn

= 10^{−5} N

≡ 1 g·cm/s^{2}

≈ 1.0197 × 10^{−6} kp

≈ 2.2481 × 10^{−6} lb_{F}

≈ 7.2330 × 10^{−5} pdl

1 kp

= 9.80665 N

= 980665 dyn

≡ g_{n}·(1 kg)

≈ 2.2046 lb_{F}

≈ 70.932 pdl

1 lb_{F}

≈ 4.448222 N

≈ 444822 dyn

≈ 0.45359 kp

≡ g_{n}·(1 lb)

≈ 32.174 pdl

1 pdl

≈ 0.138255 N

≈ 13825 dyn

≈ 0.014098 kp

≈ 0.031081 lb_{F}

≡ 1 lb·ft/s^{2}

The value of g_{n} as used in the official definition of the kilogramforce is used here for all gravitational units.

In some contexts, the term "pound" is used almost exclusively to refer to the unit of force and not the unit of mass. In those applications, the preferred unit of mass is the slug, i.e. lb_{f}·s^{2}/ft. In other contexts, the unit "pound" refers to a unit of mass. In circumstances where there may otherwise be ambiguity, the symbols "lb_{f}" and "lb_{m}" and the terms "poundsforce" and "poundsmass" can be used to distinguish.
Three approaches to mass and force units^{[6]}^{[7]}
Base

force, length, time

weight, length, time

mass, length, time

Force (F)

F = m·a = w·Template:Sfrac

F = m·Template:Sfrac = w·Template:Sfrac

F = m·a = w·Template:Sfrac

Weight (w)

w = m·g

w = m·Template:Sfrac ≈ m

w = m·g

System

BG 
GM

EE 
M

AE 
CGS 
MTS 
SI

Acceleration (a)

ft/s^{2} 
m/s^{2}

ft/s^{2} 
m/s^{2}

ft/s^{2} 
Gal 
m/s^{2} 
m/s^{2}

Mass (m)

slug 
hyl

lb_{m} 
kg

lb 
g 
t 
kg

Force (F)

lb 
kp

lb_{F} 
kp

pdl 
dyn 
sn 
N

Pressure (p)

lb/in^{2} 
at

PSI 
atm

pdl/ft^{2} 
Ba 
pz 
Pa

In the gravitational systems, the weight of the mass unit (poundmass) on Earth's surface is approximately equal to the force unit (poundforce). This is convenient because one pound mass exerts one pound force due to gravity. Note, however, unlike the other systems the force unit is not equal to the mass unit multiplied by the acceleration unit^{[8]}—the use of Newton's Second Law, F = m·a, requires another factor, g_{c}, usually taken to be 32.174049 lb_{m}·ft/lb_{f}·s^{2} = 32.174049 lb_{m}/slug.
"Absolute" systems are coherent systems of units: by using the slug as the unit of mass, the "engineering" FPS system avoids the need for such a constant. The SI is an "absolute" metric system with kilogram and meter as base units...
See also
Notes and references
 Obert, Edward F., “THERMODYNAMICS”, D.J. Leggett Book Company Inc., New York 1948; Chapter I, Survey of Dimensions and Units, pages 124.
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