What Is slug?
A slug is a unit that measures mass in some engineering systems that use feet and pounds. It is not part of the modern SI system but is still seen in some engineering work.
Definition
A slug is defined so that Newtons second law, force equals mass times acceleration, works simply when you use pounds of force and feet per second squared for acceleration.
In numbers, one slug has a mass that would need one pound of force to speed it up by one foot per second every second. This makes calculations easy in the foot pound second system.
One slug is about 14.59 kilograms. It is also about 32.17 pounds of mass. These conversion values let you switch between slug, kilogram, and pound based systems.
History / Origin
The slug was created in the late 1800s and early 1900s by engineers who worked with the British and American systems that used feet and pounds. They wanted a unit of mass that would match the way they already measured force and distance.
Before the slug, engineers often mixed pound force and pound mass, which could be confusing. The slug helped separate force and mass more clearly, similar to how the newton and kilogram work together in the SI system.
While the SI system with meters, kilograms, and newtons later became the global standard, the slug stayed in use in some English speaking engineering communities, mainly in the United States.
Symbol & Abbreviation
The slug can be written in two common ways.
- Full name. slug
- Abbreviation. sl
In many textbooks and reports you will simply see the full word slug written after the number, for example 3 slug. In more technical writing you may see the shorter form sl.
Current Use Around the World
The slug is not part of the SI system and is not widely used in everyday life. You will almost never see it on product labels or in school science outside special courses.
However, the slug still appears in some places.
- Engineering courses that use the US customary system
- Older mechanical and civil engineering textbooks
- Some industries and companies in the United States that continue to use feet and pounds in calculations
Most countries and many modern engineers prefer the SI system and measure mass in kilograms and force in newtons. Because of this, the slug is slowly becoming less common, but it is still important to understand when reading older work or certain US based materials.
Example Conversions
Here are some useful conversion facts.
- 1 slug is about 14.5939 kilograms
- 1 kilogram is about 0.06852 slug
- 1 slug is about 32.174 pounds of mass
- 1 pound of mass is about 0.03108 slug
Example 1. Convert 2 slug to kilograms.
2 slug × 14.5939 kilograms per slug is about 29.19 kilograms.
Example 2. Convert 50 kilograms to slug.
50 kilograms × 0.06852 slug per kilogram is about 3.43 slug.
Example 3. Convert 5 slug to pounds of mass.
5 slug × 32.174 pounds per slug is about 160.87 pounds.
Related Units
The slug belongs to a family of units used with feet and pounds in mechanics. These are some related units.
- Kilogram kg The main SI unit of mass, used worldwide
- Gram g A smaller SI unit of mass, one thousand grams in one kilogram
- Pound mass lbm A mass unit used in the US customary and some British systems
- Pound force lbf A force unit based on the weight of one pound mass in Earth gravity
- Newton N The SI unit of force, used with kilograms and meters
In modern physics and engineering, kilograms and newtons are preferred. In older or US based engineering, you may see slug used with pound force and feet.
FAQs
Q. What is a slug in simple words?
A. A slug is a way to measure how much matter something has, like a kilogram, but it is used in some engineering systems that work with feet and pounds.
Q. How is a slug different from a kilogram?
A. Both measure mass, but a slug belongs to the foot pound second system and a kilogram belongs to the SI system. One slug is much larger than one kilogram, about 14.59 kilograms.
Q. Why did engineers create the slug?
A. Engineers created the slug so that the formula force equals mass times acceleration would work neatly when measuring force in pound force and distance in feet.
Q. Do people still use the slug today?
A. Yes, but not many. The slug is mostly used in some US engineering courses and older books. Most modern work uses kilograms and newtons instead.
Q. Is the slug an SI unit?
A. No. The slug is not part of the SI system. The official SI unit of mass is the kilogram.
Q. How can I quickly estimate a slug from pounds?
A. You can divide the number of pounds of mass by about 32 to get a rough value in slug. For example, 64 pounds is about 2 slug.
