Slug (mass)

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 (lb_F) is exerted on it.

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

With standard gravity g_c = 9.80665 m/s², the international foot of 0.3048 m and the avoirdupois pound of 0.45359237 kg, one slug therefore has a mass of approximately 32.17405 lb_m or 14.593903 kg. At the surface of the Earth, an object with a mass of 1 slug exerts a force of about 32.17 lb_F 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", as a unit of inertia, 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.

Three approaches to mass and force units

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

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 or slinch (a portmanteau of the words slug and inch) is an inch version of the slug (1 slinch = 1 lb_f·s²/in = 12 slugs). Slang terms for the slinch include the slugette.

Metric units include the "glug" in the centimetre-gram-second system, and the "mug", "par", or "MTE" in the metre-kilogram-second system.

External links

• "What is a Slug?" on phy-astr.gsu.edu