Slugs (slug) to Kilograms (kg) conversion

What is the Slug?

The slug is the unit of mass in the British Gravitational (foot–pound–second) system of units, used chiefly in engineering and physics involving imperial units.

Definition

The slug is the mass that accelerates at 1 foot per second squared when a force of one pound-force is applied:

$$1\ \text{slug} = 14.5939\ \text{kg}$$

It follows from $1\ \text{slug} = 1\ \text{lbf} \cdot \text{s}^2/\text{ft}$, giving exactly 14.593902937206 kg. A one-slug mass therefore weighs about 32.174 pounds-force under standard gravity.

Origin and History

The slug was introduced in the early 20th century to give the imperial system a coherent mass unit consistent with Newton's second law, avoiding confusion between the pound as a unit of mass and the pound-force as a unit of force. The name was popularized by British physicist Arthur Mason Worthington.

Law and Notable Facts

The slug is not part of SI but remains in use in some U.S. and British engineering fields, especially aeronautics and ballistics. Its counterpart is the "slinch" (pound·s²/inch), which is exactly 12 times larger.

Real-World Examples and Conversions

An object weighing 32.174 lbf at standard gravity has a mass of exactly 1 slug (≈14.59 kg). A 160-pound person has a mass of roughly 4.97 slugs. One slug is close to the mass of a typical bowling-ball-and-a-half, about 14.6 kg.

What is Kilograms?

Kilogram (kg) is the base unit of mass in the International System of Units (SI). It is a fundamental unit used to measure the amount of matter in an object. Unlike units like length or time, the kilogram's definition has historically been tied to a physical artifact.

Defining the Kilogram: From Artifact to Fundamental Constant

The IPK: A Piece of Platinum-Iridium

For over a century, the kilogram was defined by the International Prototype Kilogram (IPK), a platinum-iridium cylinder stored at the International Bureau of Weights and Measures (BIPM) in France. This meant that the mass of every kilogram in the world was traceable back to this single object.

The Problem with the Artifact

The IPK was not a perfect standard. Over time, it was observed to have changed mass slightly compared to its official copies. This instability, however small, was a major concern for scientific measurements that require utmost precision.

The New Definition: Based on Planck's Constant

On May 20, 2019, the kilogram was redefined based on fundamental constants of nature. The new definition is linked to the Planck constant ($h$), a cornerstone of quantum mechanics. The Planck constant has a fixed numerical value when expressed in SI units:

$h = 6.62607015 × 10⁻³⁴ kg⋅m^2⋅s^{-1}$

By fixing the value of $h$, scientists can realize the kilogram through experiments involving quantum phenomena. This change provides a far more stable and reproducible definition than relying on a physical object. The experiment that is commonly used to realize the kilogram based on Planck's constant is called a Kibble balance.

Mass vs. Weight

It's important to distinguish between mass and weight. Mass (measured in kilograms) is the amount of matter in an object, while weight is the force exerted on that object due to gravity. Weight is measured in Newtons (N). The relationship between mass and weight is:

$Weight = mass × gravity$ or $W = mg$

Where:

• $W$ is weight (in Newtons)
• $m$ is mass (in kilograms)
• $g$ is the acceleration due to gravity (approximately 9.81 $m/s^2$ on Earth's surface)

Kilograms in Everyday Life: Examples

• Food: Groceries are often sold by the kilogram, such as fruits, vegetables, and meat.
• Human Body Weight: People often measure their body mass in kilograms.
• Construction Materials: Cement, sand, and other building materials are often bought and sold by the kilogram or metric ton (1000 kg).
• Shipping and Logistics: The weight of packages and cargo is a crucial factor in shipping costs and logistics.

Interesting Facts

• Prefixes: Kilogram is unique in that it's the only SI base unit with a prefix already in its name ("kilo," meaning 1000).
• The Kibble Balance: The Kibble balance (also known as a watt balance) is the instrument used to realize the new definition of the kilogram by linking mass to the Planck constant. For more information, you can read about the NIST's Kibble Balance.