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 (approximately 32.17405 ft/s²) but other values have been used, including 32.16 ft/s² (approximately 9.80237 m/s²).
From the acceleration of the standard gravitational field and the international avoirdupois pound, we arrive at the following definition:
|1 pound-force||≡ 0.45359237 kg × 9.80665 m/s²|
|= 4.4482216152605 N (exactly)|
Sixteen avoirdupois ounces (as a unit of mass) are equal to one avoirdupois pound (as a unit of mass). Similarly one ounce-force is equal to a sixteenth of a pound-force.
Three common, equally valid foot-pound-second (fps) systems of units for doing calculations with mass and force are summarized in the table below, which also includes the corresponding metric units.
In the "engineering" 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 = ma, requires another factor, gc, usually taken to be 32.17405 lb·ft/(lbf·s²). The "gravitational" 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.
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