Slug (slug)
The slug is an imperial unit of mass used in physics and engineering, equal to approximately 14.594 kilograms or 32.174 pounds. It is defined as the mass that is accelerated by 1 ft/s2 when a force of one pound-force is exerted on it. The slug exists primarily as a teaching tool in American physics and engineering courses that use the foot-pound-second (FPS) system.
Definition
One slug equals approximately 14.5939 kilograms or 32.174 pounds-mass. It is defined by Newton's second law in FPS units: 1 slug × 1 ft/s2 = 1 pound-force. This makes the slug the FPS system's coherent unit of mass, just as the kilogram is the SI system's base unit of mass.
History
The slug was introduced in the early 20th century to resolve the confusion between mass and weight in the British imperial system. In everyday usage, the "pound" served as both a unit of mass (pound-mass) and a unit of force (pound-force), which created problems in physics calculations. The slug provided a distinct unit of mass for the FPS system, analogous to the kilogram in SI. British physicist Arthur Mason Worthington is credited with first using the term around 1905. The slug gained prominence in American engineering education but never entered everyday use.
Common Uses
The slug appears in American physics and mechanical engineering textbooks that use the FPS system. It is used in aerospace engineering calculations involving imperial units, particularly for aircraft weight and balance, rocket thrust, and structural analysis. Some older NASA documentation uses slugs. Outside of education and legacy engineering contexts, the slug has virtually no everyday applications.
Did You Know? Facts About Slug
- A slug of mass on Earth weighs about 32.174 pounds - roughly the weight of a medium dog.
- The slug was invented to solve the confusing dual use of "pound" as both a mass and a force unit in imperial systems.
- Despite its name, the slug has nothing to do with the garden creature - it likely comes from the word "sluggish," meaning slow to accelerate.
- Most American physics students encounter the slug in their first engineering mechanics course and rarely use it again.
- In SI, the kilogram serves the role the slug was designed for, but without the confusion, since SI uses the newton as its force unit.