HOME

TheInfoList



OR:

The foot–pound–second system or FPS system is a system of units built on three fundamental units: the
foot The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg mad ...
for
length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Inte ...
, the (avoirdupois) pound for either
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...
or
force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...
(see below), and the
second The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ea ...
for
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...
..


Variants

Collectively, the variants of the FPS system were the most common system in technical publications in English until the middle of the 20th century. Errors can be avoided and translation between the systems facilitated by labelling all physical quantities consistently with their units. Especially in the context of the FPS system this is sometimes known as the Stroud system after
William Stroud William is a male given name of Germanic origin.Hanks, Hardcastle and Hodges, ''Oxford Dictionary of First Names'', Oxford University Press, 2nd edition, , p. 276. It became very popular in the English language after the Norman conquest of ...
, who popularized it.


Pound as mass unit

When the pound is used as a unit of mass, the core of the coherent system is similar and functionally equivalent to the corresponding subsets of the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...
(SI), using metre, kilogram and second (MKS), and the earlier
centimetre–gram–second system of units The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units ...
(CGS). This system is often called the Absolute English System. In this sub-system, the unit of
force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...
is a derived unit known as the poundal. :\mathrm = \mathrm. The international standard symbol for the pound as unit of mass rather than force is lb. Everett (1861) proposed the metric
dyne The dyne (symbol: dyn; ) is a derived unit of force specified in the centimetre–gram–second (CGS) system of units, a predecessor of the modern SI. History The name dyne was first proposed as a CGS unit of force in 1873 by a Committee of ...
and
erg The erg is a unit of energy equal to 10−7joules (100 nJ). It originated in the Centimetre–gram–second system of units (CGS). It has the symbol ''erg''. The erg is not an SI unit. Its name is derived from (), a Greek word meaning 'work' o ...
as the units of force and energy in the FPS system. Latimer Clark's (1891) "Dictionary of Measures" contains ''celo'' (acceleration), ''vel'' or ''velo'' (velocity) and ''pulse'' (momentum) as proposed names for FPS absolute units.


Pound as force unit

The technical or gravitational FPS systemJ. M. Coulson, J. F. Richardson, J. R. Backhurst, J. H. Harker: Coulson & Richardson's Chemical Engineering: Fluid flow, heat transfer, and mass transfer.
/ref> or British gravitational system is a coherent variant of the FPS system that is most common among engineers in the United States. It takes the
pound-force The pound of force or pound-force (symbol: lbf, sometimes lbf,) is a unit of force used in some systems of measurement, including English Engineering units and the foot–pound–second system. Pound-force should not be confused with pou ...
as a fundamental unit of force instead of the pound as a fundamental unit of mass. In this sub-system, the unit of mass is a derived unit known as the
slug Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a ...
. :\mathrm = \mathrm. In the context of the gravitational FPS system, the pound-force (lbf) is sometimes referred to as the pound (lb).


Pound-force as force unit and pound-mass as mass unit

Another variant of the FPS system uses both the pound-mass and the pound-force, but neither the slug nor the poundal. The resulting system is sometimes also known as the English engineering system. Despite its name, the system is based on United States customary units of measure; it is not used in England.


Other units


Molar units

The unit of substance in the FPS system is the pound-mole (lb-mol) = . Until the SI decided to adopt the gram-mole, the mole was directly derived from the mass unit as (mass unit)/(atomic mass unit). The unit (lbf⋅s2/ft)-mol also appears in a former definition of the atmosphere.


Electromagnetic units

The electrostatic and electromagnetic systems are derived from units of length and force, mainly. As such, these are ready extensions of any system of containing length, mass, time. Stephen Dresner gives the derived electrostatic and electromagnetic units in both the foot–pound–second and foot–slug–second systems. In practice, these are most associated with the centimetre–gram–second system. The 1929 "International Critical Tables" gives in the symbols and systems fpse = FPS electrostatic system and fpsm = FPS electromagnetic system. Under the conversions for charge, the following are given. The CRC Handbook of Chemistry and Physics 1979 (Edition 60), also lists fpse and fpsm as standard abbreviations. ; Electromagnetic FPS (EMU, stat-) : 1 fpsm unit = 117.581866 cgsm unit (Biot-second) ; Electrostatic FPS (ESU, ab-) : 1 fpse unit = 3583.8953 cgse unit (Franklin) : 1 fpse unit = 1.1954588×10−6 abs coulomb


Units of light

The ''
candle A candle is an ignitable wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a fragrance. A candle can also provide heat or a method of keeping time. A person who makes candle ...
'' and the ''foot-candle'' were the first defined units of light, defined in the Metropolitan Gas Act (1860). The foot-candle is the intensity of light at one foot from a ''standard candle''. The units were internationally recognized in 1881, and adopted into the metric system.


Conversions

Together with the fact that the term "weight" is used for the gravitational force in some technical contexts (physics, engineering) and for mass in others (commerce, law),NIST Federal Standard 376B
, p. 13.
and that the distinction often does not matter in practice, the coexistence of variants of the FPS system causes confusion over the nature of the unit "pound". Its relation to international metric units is expressed in kilograms, not newtons, though, and in earlier times it was defined by means of a mass prototype to be compared with a two-pan balance which is agnostic of local gravitational differences. In July 1959, the various national foot and avoirdupois pound standards were replaced by the international foot of precisely and the international pound of precisely , making conversion between the systems a matter of simple arithmetic. The conversion for the poundal is given by 1 pdl = 1 lb·ft/s2 = (precisely). To convert between the absolute and gravitational FPS systems one needs to fix the standard acceleration ''g'' which relates the pound to the pound-force. :1\,\text = 1\,\text\cdot g While ''g'' strictly depends on one's location on the Earth surface, since 1901 in most contexts it is fixed conventionally at precisely ''g''0 =  ≈ .


See also

*
Metre–tonne–second system of units The metre–tonne–second or MTS system of units is a system of physical units. It was invented in France, hence the unit names ''sthène'' and ''pièze'', and became its legal system between 1919 and 1961 ("décret" 5 May 1961, "Journal Officie ...
(MTS) * FFF system


References

Systems of units Imperial units Customary units of measurement in the United States {{United States Customary Units