Slug (mass)

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The slug is a unit of mass associated with Imperial units and United States customary units. It is a mass that accelerates by 1 ft/s2 when a force of one pound (lbF) is exerted on it.

1\,\text{slug} =1\,\frac{\text{lb}_F\cdot\text{s}^2}{\text{ft}}
1\,\text{lb}_F = 1\,\frac{\text{slug}\cdot\text{ft}}{\text{s}^2}

One slug has a mass of 32.174049 lbm or 14.593903 kg based on standard gravity, the international foot, and the avoirdupois pound.[1] At the surface of the Earth, an object with a mass of 1 slug exerts a force of approximately 32.2 lbF or 143 N.[2][3]


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.[4]

The name "slug" was coined before 1900 by British physicist Arthur Mason Worthington,[5] 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 mass and force units[6][7]
Base force, length, time weight, length, time mass, length, time
Force (F) F = ma = wa/g F = ma/gc = wa/g F = ma = wa/g
Weight (w) w = mg w = mg/gcm w = mg
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

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[edit]

The blob is the inch version of the slug (1 blob = 1 lbf·s2/in = 12 slugs)[1] or equivalent to 175.126 kg. This unit is also called slinch (a portmanteau of the words slug and inch).[8][9] Similar terms include slugette,[10] and a snail.[11]

Similar metric units include the "glug" in the centimetre-gram-second system, and the "mug", "par", or "MTE" in the metre-kilogram-second system.[12]


  1. ^ a b Shigley, Joseph E. and Mischke, Charles R. Mechanical Engineering Design, Sixth ed, pp. 31–33. McGraw Hill, 2001. ISBN 0-07-365939-8.
  2. ^ Beckwith, Thomas G., Roy D. Marangoni, et al. Mechanical Measurements, Fifth ed, pp. 34-36. Addison-Wesley Publishing, 1993. ISBN 0-201-56947-7.
  3. ^ Shevell, R.S. Fundamentals of Flight, Second ed, p. xix. Prentice-Hall, 1989.
  4. ^ [1].
  5. ^ Worthington, Arthur Mason (1900). Dynamics of Rotation: An Elementary Introduction to Rigid Dynamics (3rd ed.). Longmans, Green, and Co. p. 9. 
  6. ^ Michael R. Lindeburg (2011). Civil Engineering Reference Manual for the Pe Exam. Professional Publications. ISBN 1591263417. 
  7. ^ Wurbs, Ralph A, Fort Hood Review Sessions for Professional Engineering Exam (PDF), retrieved October 26, 2011 
  8. ^ Slug. DiracDelta Science & Engineering Encyclopedia
  9. ^ "1 blob". Wolfram Alpha Computational Knowledge Engine. Retrieved 27 October 2011. 
  10. ^ Celmer, Robert. Notes to Accompany Vibrations II. Version 2.2. 2009.
  11. ^ [2]
  12. ^ Cardarelli, François (1999). Scientific Units, Weights and Measures. Springer. pp. 358, 377. ISBN 1-85233-682-X. 

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