Foot-pound-second (FPS) Systems of 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. lbf·s2/ft. In other contexts, the unit "pound" refers to a unit of mass. In circumstances where there may otherwise be ambiguity, the symbols "lbf" and "lbm" and the terms "pounds-force" and "pounds-mass" can be used to distinguish.
| Base | force, length, time | weight, length, time | mass, length, time | |||||
|---|---|---|---|---|---|---|---|---|
| Force (F) | F = m·a = w·a/g | F = m·a/gc = w·a/g | F = m·a = w·a/g | |||||
| Weight (w) | w = m·g | w = m·g/gc ≈ m | w = m·g | |||||
| System | BG | GM | EE | M | AE | CGS | MTS | SI |
| Acceleration (a) | ft/s2 | m/s2 | ft/s2 | m/s2 | ft/s2 | Gal | m/s2 | m/s2 |
| Mass (m) | slug | hyl | lbm | kg | lb | g | t | kg |
| Force (F) | lb | kp | lbF | kp | pdl | dyn | sn | N |
| Pressure (p) | lb/in2 | at | PSI | atm | pdl/ft2 | Ba | pz | Pa |
In the gravitational systems, the weight of the mass unit (pound-mass) on Earth's surface is approximately equal to the force unit (pound-force). 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—the use of Newton's Second Law, F = m·a, requires another factor, gc, usually taken to be 32.174049 lbm·ft/lbf·s2 = 32.174049 lbm/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.
Read more about this topic: Pound (force)
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