Kilograms (kg) to Long Tons (long-ton) conversion

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.

What is the Long Ton?

The long ton is a unit of mass in the British imperial system, historically used for weighing bulk commodities such as coal, ore, and shipping cargo. It is equal to 2,240 pounds and remains familiar in the United Kingdom and in maritime contexts.

Definition

The long ton is defined as exactly 2,240 pounds avoirdupois. Since one pound equals 0.45359237 kg exactly, its SI value is:

$$1\ \text{long-ton} = 1016.05\ \text{kg}$$

Equivalently, the long ton equals 20 hundredweight (cwt) of 112 lb each, distinguishing it from the 2,000 lb US short ton and the 1,000 kg metric tonne.

Origin and History

The long ton descends from the medieval English "tun," a large cask used to ship wine, whose full weight was standardized in trade. The avoirdupois system fixed the hundredweight at 112 pounds (eight stone of 14 lb), and twenty of these made the ton, giving 2,240 lb. This "long" hundredweight and ton became the standard for British commerce and were carried throughout the Commonwealth and colonial trade.

Law and Notable Facts

The long ton is the British/imperial ton, contrasted with the 2,000 lb US short ton and the 1,000 kg metric tonne (megagram). It is legally recognized in the United Kingdom, though the metric tonne has largely replaced it for official trade. Notably, the long ton (1016 kg) sits remarkably close to the metric tonne (1000 kg), differing by only about 1.6%, which is why "ton" and "tonne" are often used loosely as near-equivalents in shipping.

Real-World Examples and Conversions

• A long ton equals 1016.05 kg, about 16 kg heavier than a metric tonne.
• Ship displacement and cargo were traditionally quoted in long tons; a 10,000 long ton vessel carries roughly 10,160 metric tonnes.
• One long ton (2,240 lb) is 240 lb heavier than one US short ton (2,000 lb), a 12% difference.
• A long ton of coal (1016 kg) is a typical unit still cited in older British mining and heating records.