Pound-force

Definitions

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/s² (32.174 049 ft/s²).

The acceleration of the standard gravitational field (g_n) and the international avoirdupois pound (lb_m) define the pound-force as: :

This definition can be rephrased in terms of the slug, a mass unit equivalent to 32.17405 lb_m. A pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s², so: :$1\backslash ,\backslash mathrm\{lb\}\_F\; =\; 1\backslash ,\backslash mathrm\{slug\}\; \backslash cdot\; 1\backslash ,\backslash mathrm\{\backslash tfrac\{ft\}\{s^2\}\}$

Conversion to other units

Foot-pound-second systems of units

In the "gravitational" foot-pound-second (fps) system, the weight of the mass unit (pound-mass) on Earth's surface is approximately equal to the force unit (pound-force). The price for this convenience is that the force unit is not equal to the mass unit multiplied by the acceleration unit—the use of Newton's Second Law, F = m·a, requires another factor, g_c, usually taken to be 32.17405 lb·ft/(lb_f·s²) = 32.17405 lb_m/slug. The "engineering" fps system is a coherent system of units: by using the slug as the unit of mass, it avoids the need for such a constant. The "absolute" system is similarly coherent; the SI units are those of the "absolute" metric system.

See also

Notes and references

Category:Units of force Category:Imperial units Category:Customary units in the United States