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The slug is a derived unit of mass in the weight-based system of measures, most notably within the British Imperial measurement system and in the United States customary measures system. Systems of measure either define mass and derive weights or define a base weight and derive a mass unit. A slug is defined as the mass that is accelerated by 1 ft/s2 when a force of one pound (lbf) is exerted on it.

$1\,{\text{slug}}=1\,{\frac {{\text{lbf}}{\cdot }{\text{s}}^{2}}{\text{ft}}}\quad \Longleftrightarrow \quad 1\,{\text{lbf}}=1\,{\frac {{\text{slug}}{\cdot }{\text{ft}}}{{\text{s}}^{2}}}$ One slug has a mass of 32.174049 lb or 14.593903 kg based on standard gravity, the international foot, and the avoirdupois pound. At the surface of the Earth, an object with a mass of 1 slug exerts a force downward (definition of weight) of approximately 32.2 lbf or 143 N.

## History

The slug is part of a subset of units known as the gravitational FPS system, one of several such specialized systems of mechanical units developed in the late 19th and the 20th century. Geepound was another name for this unit in early literature.

The name "slug" was coined before 1900 by British physicist Arthur Mason Worthington, but it did not see any significant use until decades later. A 1928 textbook says:

No name has yet been given to the unit of mass and, in fact, as we have developed the theory of dynamics no name is necessary. Whenever the mass, m, appears in our formulae, we substitute the ratio of the convenient force-acceleration pair (w/g), and measure the mass in lbs. per ft./sec.2 or in grams per cm./sec.2.

— Noel Charlton Little, College Physics, Charles Scribner's Sons, 1928, p. 165.
Three approaches to units of mass and force or weight
Base Force Weight Mass
2nd law of motion m = F/a F = Wa/g F = ma
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 pound-mass kilogram pound gram tonne kilogram
Force (F),
weight (W)
pound kilopond pound-force kilopond poundal dyne sthène newton
Pressure (p) pound per square inch technical atmosphere pound-force per square inch atmosphere poundal per square foot barye pieze pascal

The slug is listed in the Regulations under the Weights and Measures (National Standards) Act, 1960. This regulation defines the units of weights and measures, both regular and metric, in Australia.

## Similar units

The blob is the inch version of the slug (1 blob= 1 lbf⋅s2/in= 12 slugs) or equivalent to 175.126 kg. This unit is also called slinch (a portmanteau of the words slug and inch). Similar terms include slugette, and a snail.

Similar metric units include the glug in the centimetre–gram–second system, and the mug, par, or MTE in the metre–kilogram–second system.