The pound-force 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 low-precision measurements, for which small changes in
Earth's gravity (which varies from equator to pole by up to half a percent) can safely be neglected. The 20th century, however, brought the need for a more precise definition, requiring a standardized value for acceleration due to gravity.
Product of avoirdupois pound and standard gravity The pound-force is the product of one
avoirdupois pound (
exactly ) and the
standard acceleration due to gravity, approximately . The standard values of acceleration of the standard gravitational field (
gn) and the
international avoirdupois pound (lb) result in a pound-force equal to (). \begin{align} 1\,\text{lbf} &= 1\,\text{lb} \times g_\text{n} \\ &= 1\,\text{lb} \times 9.80665\,\tfrac{\text{m}}{\text{s}^2} / 0.3048\,\tfrac{\text{m}}{\text{ft}}\\ &\approx 1\,\text{lb} \times 32.174049\,\mathrm{\tfrac{ft}{s^2}}\\ &\approx 32.174049\,\mathrm{\tfrac{ft {\cdot} lb}{s^2}} \\ 1\,\text{lbf} &= 1\,\text{lb} \times 0.45359237\,\tfrac{\text{kg}}{\text{lb}} \times g_\text{n} \\ &= 0.45359237\,\text{kg} \times 9.80665\,\tfrac{\text{m}}{\text{s}^2}\\ &= 4.4482216152605\,\text{N} \end{align} This definition can be rephrased in terms of the
slug. A slug has a mass of 32.174049 lb. A pound-force is the amount of force required to accelerate a slug at a rate of , so: \begin{align} 1\,\text{lbf} &= 1\,\text{slug} \times 1\,\tfrac{\text{ft}}{\text{s}^2} \\ &= 1\,\tfrac{\text{slug} \cdot \text{ft}}{\text{s}^2} \end{align} == Conversion to other units ==