5 Fascinating Facts About Mass (Not Weight)

People often mix up mass and weight, but they're actually distinct. Weight is the force that gravity exerts on an object, while mass refers to the amount of matter in an object and its resistance to changes in motion. The mass vs weight debate may seem tricky, but there are plenty of fun facts to help clarify the difference.

Climb to the top of Mount Everest, and you'll shed a bit of weight—not from exercise, but because gravity weakens at higher altitudes. A person standing at sea level weighs a tad more than they would at the mountain's summit. That’s weight in action, as it fluctuates, while mass remains unchanged.

Mass is a whole different animal, and it's surprisingly tough to define. Imagine you're still at the top of Mount Everest with your backpack. While you might have lost a bit of weight due to the altitude, your backpack’s mass remains exactly the same. It might feel lighter because gravity’s pull is weaker, but the mass stays constant. In this article, we’ll dive into five intriguing facts about mass, so you’ll never mix it up with weight again.

1. There Exists a Unit of Mass Known as a 'Slug'

The term "slug" refers to a unit of mass in the U.S. Customary Units system, and also in the lesser-known British Imperial System. For most of the world, however, the kilogram is the standard unit of mass, with one thousand kilograms equaling a metric ton. Kilograms are part of the International System of Units, also called the metric system. While most countries use them daily, Americans are more accustomed to the U.S. Customary Units, especially when using a spring balance.

You might think the U.S. Customary system’s equivalent to the kilogram is the pound. But pounds are actually units of weight. Both the U.S. Customary and British Imperial Systems use a different unit for mass called a "slug." (On Earth, one slug equals about 32.2 pounds of downward force, or 14.60 kilograms.) Despite this, the term is rarely used in everyday conversation, and most people aren’t familiar with it. Too bad, right? Imagine fans at WrestleMania cracking slug jokes at ringside.

2. The Scientist Who Discovered the Law of Conservation of Mass Was Beheaded

"In every process," wrote the renowned chemist Antoine-Laurent Lavoisier, "an equal amount of matter exists before and after the process." In simpler terms, an object’s mass can’t be created or destroyed. This concept became known as the Law of Conservation of Mass. Lavoisier’s late 18th-century experiments were key to uncovering this principle.

Other scientists welcomed his discovery, but Lavoisier's career came to a tragic end. Quite literally. When he wasn't experimenting with water decomposition or intentionally rusting metals, Lavoisier was involved in tax collection for the French government. This led to his execution by guillotine in 1794, after being accused of "conspiracy against the people of France" by revolutionary forces.

3. It's Part of the Most Famous Equation Ever Written

Naturally, we’re referring to E = mc². Put into simple terms, this equation tells us that energy (E) equals mass (m) multiplied by the speed of light (c) squared. Albert Einstein explored the reverse side of this equation in his groundbreaking paper, published on Sept. 27, 1905, when he was just 26 years old.

As Einstein famously said, "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."

Every physical property that has mass also contains inherent energy. Einstein's groundbreaking theory explains why atoms are slightly less massive than the combined weight of their protons, neutrons, and electrons. This mass-energy relationship is also responsible for the tremendous destructive force of atomic bombs.

4. Light Is Made of "Massless Particles"

Photons are the basic particles that make up light, and they are often described as "massless." This is because the mass of an object is affected by its speed, and light travels at an extraordinary speed.

When physicists discuss the mass of an object or particle, they are usually referring to its rest mass — the mass it has when it's not in motion. Neutrons, protons, and electrons all have rest masses, but photons do not. The same goes for gluons, another type of subatomic particle.

5. Earth and the Moon Share a Common "Center of Mass"

Earth’s mass is 81 times greater than the moon’s, and this difference significantly impacts their relationship — gravity aside. When multiple celestial bodies like moons, planets, and suns orbit each other, they actually revolve around a shared center of mass. This common point, called the barycenter, is determined by the masses involved.

If two objects with identical mass begin orbiting one another, their barycenter will lie directly between them. However, because Earth is so much larger than the moon, the Earth-moon barycenter is positioned deep within Earth itself. Yet, Earth still orbits this point, just like the moon does.