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The slug is a unit of mass associated with Imperial units. It is a mass that accelerates by 1 ft/s2 when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of 32.17405 pound-mass or 14.5939 kg.[1]

 1\ \mbox{slug} =1 \cfrac{\mbox{lbf}\cdot\mbox{s}^2}{\mbox{ft}}

(in terms of mass x acceleration) as

 1\ \cfrac{\mbox{slug}\cdot\mbox{ft}}{\mbox{s}^2} = \mbox{lbf}

The slug is part of a subset of coherent units known as the gravitational foot-pound-second system (FPS), one of several such specialized systems of mechanical units developed in the late 19th and the 20th century. See the article poundal for an explanation of the problem such English units were introduced to solve.

The slug was first used in 1902 by Arthur Mason Worthington (1852–1916) in Dynamics of Rotation (OED), but it didn't 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".[2]

Another name for this unit in early literature is the geepound.[3]

The unit slinch (derived from the words slug-inch [4]) is an inch version of the slug (1 slinch = 1 lbf·s2/in = 12 slugs).[5] The unit blob (bl) is also an inch version of the slug (1 lbf·s2/in).[6]

Three approaches to mass and force units
System Gravitational Engineering Absolute
Force (F) F = m·a F = m·a/gc = w·a/g F = m·a
Weight (w) w = m·g w = m·g/gc ≈ m w = m·g
Units English Metric English Metric English Metric
Acceleration (a) ft/s2 m/s2 ft/s2 m/s2 ft/s2 m/s2
Mass (m) slug hyl pound-mass kilogram pound kilogram
Force (F) pound kilopond pound-force kilopond poundal newton

Source: Mechanical Measurements[7]

References

  1. ^ Shigley, Joseph E. and Mischke, Charles R. Mechanical Engineering Design, Sixth ed. McGraw Hill, pp. 18–19, 2006. ISBN 0-07-365939-8.
  2. ^ Noel Charlton Little, College Physics, Charles Scribner’s Sons, 1928, p. 165.
  3. ^ http://www.unit2unit.eu/lb_gee.html
  4. ^ Miscellaneous
  5. ^ Slug - DiracDelta Science & Engineering Encyclopedia
  6. ^ Robert L. Norton, Worcester Polytechnic Institute, Machine Design: An Integrated Approach, Third Edition, Prentice Hall ISBN 0-13-048190-8
  7. ^ Beckwith, Thomas G., Marangoni, Roy D., et al. Mechanical Measurements, Fifth ed. Addison-Wesley Publishing, 1993. ISBN 0-201-56947-7.

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