Mass and weight tend to be used interchangeably nowadays, but as you may remember from taking physics tuition, they’re actually quite different. Weight measures the pull of gravity on an object, whereas mass represents how much matter makes up an object and its resistance to changes in motion. While the concepts of mass and weight can seem a bit complex, the differences are easy to grasp with some fascinating facts.
For instance, if you scale Mount Everest, you’re bound to shed some pounds but not because of the thousands of calories you need to expend to get to the top. A person at sea level actually weighs a bit more than if they were at the peak of the mountain, as weight varies with gravity’s pull. Mass, however, remains constant regardless of location.
Let’s continue with this example and say you’re still atop Mount Everest with a full backpack. Even though you weigh less due to the altitude, your backpack’s mass stays the same. While it may not feel as heavy because of the reduced gravitational force, it still retains the same amount of matter. With that said, let’s dive right into some fun facts that’ll help you distinguish it from weight once and for all.
1. It’s in the Most Famous Equation in History
The renowned equation E = mc² by Albert Einstein is something that virtually everyone is familiar with, and it means that energy (E) is equal to mass (m) multiplied by the square of the speed of light (c²). He explains that it followed from the special theory of relativity wherein energy and mass are both but different manifestations of the same thing.
This conception which is somewhat unfamiliar for the average mind reveals that all objects with mass inherently contain energy. In fact, it explains why the mass of an atom is slightly less than the combined mass of its protons, neutrons, and electrons, due to the energy that binds these particles together. This same mass-energy relationship also underpins the immense power released in atomic bombs.
2. There’s an Official Unit of Mass
Slug is the official unit of mass in the British Imperial System and U.S. Customary Units system. While much of the world measures mass in kilograms (with 1,000 kilograms equalling a metric ton), Americans often rely on their own units, especially when using devices like spring balances.
Surprisingly, the closest U.S. Customary counterpart to the kilogram isn’t the pound, which is technically a unit of weight, not mass. Instead, both systems use a lesser-known unit of mass called the “slug.” One slug on is roughly equivalent to about 14.60 kg or 32.2 lbs of downward force.
3. Earth and Its Moon Share a Common “Centre of Mass”
Earth’s mass is 81 times greater than the Moon’s, a difference that significantly shapes their relationship beyond the pull of Earth’s gravity. When two celestial bodies, such as planets and moons, orbit each other, they don’t revolve around one object; instead, they orbit a shared centre of mass known as the barycentre. The location of this point depends on the masses involved.
If two objects of equal mass were to orbit each other, their barycentre would be exactly halfway between them. However, because Earth is much more massive, the Earth-Moon barycentre is located deep within Earth itself. Despite this, both Earth and the Moon revolve around this point, making Earth’s orbit around the barycentre a subtle but fascinating part of their cosmic dance.
4. Light is made up of “Massless Particles”
Photons, the fundamental particles of light, are described as “massless.” Here’s why: the mass of an object increases with its speed, which can complicate things when measuring fast-moving particles.
To avoid confusion, physicists often refer to a particle or body’s mass as “rest mass,” which is the mass it has when it isn’t moving. Particles like neutrons, electrons, and protons possess rest masses, but photons do not—because they’re never at rest. Photons, along with another kind of massless particle called gluons, always travel at the speed of light, making rest mass irrelevant for them. This unique quality allows light to behave in ways distinct from other matter.
Conclusion
Mass may seem like a straightforward concept, but as we’ve explored, it holds countless fascinating surprises—from invisible particles and improbable units to cosmic interactions and profound scientific principles. These lesser-known facts remind us that even the most familiar ideas can reveal extraordinary insights when we look a little closer. So next time you encounter a physics concept, remember: there’s always more than meets the eye.
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