5 Hugely Fun Facts About Mass (Not Weight)

The "slug" is technical terminology of the U.S. Customary Units system and the less popular British Imperial System. For most of the world, the preferred unit of mass is the kilogram, one thousand of which equal a metric ton. Kilograms belong to the International System of Units, also known as the metric system. While they're everyday terms in other countries, Americans tend to be more reliant on U.S. Customary Units when using a spring balance.

Now you might assume this system's answer to the kilogram is the pound. Yet pounds are technically units of weight. Both the U.S. Customary and British Imperial Systems measure mass with a different unit called a "slug." (On Earth, one slug is equal to about 32.2 pounds of downward force, or 14.60 kilograms.) Even so, it rarely comes up in casual conversations and most users aren't familiar with the term. That's a real pity; imagine WrestleMania fans cracking slug jokes by the ringside.

"In every operation," wrote the great chemist Antoine-Laurent Lavoisier, "an equal quantity of matter exists before and after the operation." Put another way, an object's mass can neither be created nor destroyed. This principle has been named the Law of Conservation of Mass. Lavoisier's experiments in the late 18th century brought this idea to light.

Fellow scientists embraced his findings, but Lavoisier's career was cut short. Literally. When he wasn't decomposing water or making rust on purpose, Lavoisier helped collect taxes for the French government. That got him guillotined in 1794, after he was charged with "conspiracy against the people of France" by revolutionary forces.

Of course, we're talking about E = mc². Stated in plain English, it says energy (E) equals mass (m) times the speed of light (c) squared. Albert Einstein discussed the flip-side of this equation in a classic paper published Sept. 27, 1905. By the way, he was only 26 years old at the time.

Said Einstein, "It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing — a somewhat unfamiliar conception for the average mind."

So there's an inherent energy found in all physical properties that possess mass. Einstein's breakthrough explains why every atom is slightly less massive than the sum of its parts (namely the protons, neutrons, and electrons which comprise it). And the same energy/mass relationship he observed accounts for the destructive power of atomic bombs.

Photons are the fundamental particles of light. Experts describe them as being "massless." You see, the speed of a traveling object always changes its mass.

Because that can complicate scientific discussions, when physicists talk about the mass of a given body or particle, what they're usually referring to is its rest mass. Basically, that's the mass it possesses when its velocity is equal to zero. Neutrons, protons, and electrons all have rest masses — but photons don't! Neither do gluons, another type of subatomic particle.

Earth's mass is 81 times greater than the moon's, and the disparity has a profound effect on their relationship -- Earth's gravity notwithstanding. When you've got two or more heavenly bodies — like moons, planets, and suns — orbiting each other, they're really revolving around a common center of mass. Called the barycenter, its location depends on the participants.

If two objects with the exact same mass start orbiting one another, their barycenter will be situated directly between them. But since Earth's surface is so much larger than the moon, the Earth-moon barycenter is located deep inside our home world. And yet the Earth still revolves around it, just like the moon does.