SI Unit: Kilograms (kg)

## What Is Mass?

Mass is a measurement of how much matter ("stuff") an object contains. Physicists define mass by measuring how much an object resists a force that is applied to it. For example, if you blow on a feather, it will easily fly away—but try the same thing with an airplane, and our guess is it won't move very far! The SI or metric unit for mass is the kilogram (kg), which is equal to 1,000 grams (g).

## How Is Mass Measured?

Mass is typically measured with a mass scale, or mass balance. A balance is usually made of a moving bar, supported at a point in the middle, with pans on each end.

To measure an object's mass, it is placed in one pan, and standard masses, called weights, are added to the other pan. Weights are added or removed until the bar is level, indicating that both pans have the same mass. The mass of the object is then calculated by adding up the masses of the weights on the opposite pan. Today, electronic scales are also commonly used, which provide quick and accurate measurements of mass.

## How Is Mass Different Than Weight?

Although mass and weight can both be measured with a scale, they're actually different! Weight is actually the measurement of the force of gravity on an object. When we weigh ourselves on Earth, we're actually measuring the Earth's gravitational pull on ourselves.

Mass, on the other hand, is an unchanging property of an object. What this means is that, while an astronaut will weigh less on the Moon, since the Moon has a weaker gravitational pull than the Earth, the same astronaut will have the same mass, no matter where they travel to. Mass and weight are, however, related, since the force of gravity is stronger on objects with greater mass.

## Top Facts About Mass for Kids

• #### 1. Mass can never be created or destroyed in the universe... sort of.

In the 1600s a French chemist, Jean Rey, came up with an idea that became known as the law of conservation of mass. This law says that if you have a closed container (like a sealed test tube... or even the entire universe), the mass of everything before a reaction will be the same as the mass of everything afterwards.

In 1905, however, Einstein came along and proved that mass and energy are the same. With Einstein's discoveries, physicists discovered they could convert matter into energy with nuclear reactions. Later, with new discoveries in a complicated area of physics called quantum mechanics, things got even more complicated. Today, physicists have even made particles disappear by shooting them into “anti-particles.” Now scientists know that it’s more accurate to say that “mass-energy” is conserved.

• #### 2. The word "mass" comes from the Greek word maza, meaning "barley" or "cake."

In Latin, maza became massa, meaning a lump or piece of dough. Also sharing the same root are the words "massage," which came from the action of kneading dough, and masa in Spanish, which also means dough.

• #### 3. While the SI, or metric, unit for mass is the kilogram (kg), the US and imperial systems use a unit called the "slug."

One slug is equal to about 14.6 kg, which is about the typical mass of a large watermelon. Unlike what you might think, it didn't get its name from the slimy creature sometimes seen slowly sliding across the sidewalk, but from an old-fashioned word for a "solid block of metal." The slug isn't that common, though, and is mostly only used by engineers working on projects like aircraft or other vehicles.

• #### 4. Neutron stars are so dense that 1 mL worth of their material has about the same mass as a mountain on Earth.

Neutron stars are the second most dense objects in the universe after black holes. (Black holes have no physical size, which means they have infinite density). Neutron stars are formed when a massive star explodes in a supernova, leaving behind a tiny, incredibly dense core. In fact, 1 mL (.20 tsp) of a neutron star would have an approximate mass of 4 x 10^11 kilograms (4 with 11 zeros) or about 75 times the mass of the Great Pyramid of Giza.

• #### 5. Time itself actually moves slower the closer you are to objects with great amounts of mass.

Time dilation might sound like something from a science fiction movie, but it’s actually real! Einstein explained it with his theory of relativity. Time dilation means that time itself actually moves slower near a planet, star, or other object than it does in empty outer space. Scientists have even done experiments with atomic clocks to prove that this is true—the clocks tick slower in stronger gravitational fields.

One extreme example of time dilation is in black holes. Black holes have huge amounts of mass, even though they technically have no size. The gravity around a black hole is so strong that time slows to a crawl. Anything that gets too close to a black hole gets sucked in and destroyed. If you could stand on the edge of a black hole (which you can’t because you’d get pulled in), you would see the rest of the universe moving faster than normal!

• #### 6. If you were to take all of the matter in the universe, and add it up, you'd only get about 5% of the universe's total mass.

This might sound really weird, and that's because it is! It turns out, 95% of the total mass of the universe is made up of dark matter and dark energy, which scientists are still trying to understand. Dark matter is a type of matter that doesn't interact with light or other forms of electromagnetic radiation, so it can't be seen directly. But scientists can still detect it by observing the way it affects the motion of galaxies and galaxy clusters.

While we don't know exactly what dark matter is made of, scientists think it might be made up of particles that are yet to be discovered. Understanding dark matter and dark energy is one of the biggest mysteries in science today, and could help us learn more about the origins and fate of the universe.

• #### 7. The smallest known particle with mass is the neutrino, which has a mass about a million times less than that of an electron.

These tiny particles were originally thought by physicists to not have any mass at all, but experiments have since shown that they do have a very small mass. Neutrinos interact very weakly with other matter and weirdly enough can travel long distances through solid objects without being stopped or absorbed.

Neutrinos are created in many natural processes such as nuclear reactions in the Sun, supernova explosions, and cosmic ray interactions in the Earth's atmosphere. Scientists study these mysterious particles to learn more about the fundamental properties of matter and the workings of the Universe.

• #### 8. The largest "object" in the known universe has a mass about 10 quadrillion times that of the Sun.

The Hercules–Corona Borealis Great Wall, a galaxy supercluster or group of hundreds or thousands of galaxies held together by gravity, has an estimated mass of around 10 quadrillion times the mass of the Sun or 20 billion trillion kilograms (2 with 47 zeros after it). Astronomers estimate it would take approximately 10 billion light years to cross the entire structure.

To estimate its mass, astronomers usually use two techniques known as the virial theorem and gravitational lensing. The virial theorem looks at how the galaxies in the cluster move to estimate the total mass of the cluster, while gravitational lensing is when light is bent around massive objects like galaxy clusters.

• #### 9. Physicists believe mass comes from the interaction of particles with the Higgs field, an invisible forcefield, similar to a magnetic or electrical field.

You can think of the Higgs field like an invisible ocean that fills all of space. Whenever a particle moves through the field, it interacts with it, gaining mass. Imagine walking through a swimming pool—the water around you slows you down and makes it harder to move. A boat on the other hand is designed to glide easily through the water. In the same way, the Higgs field slows down some particles and gives them mass, while affecting others less.

First theorized in 1964 by a group of physicists, it was only in 2012 that the Higgs field's existence was proven by the discovery of a particle called the Higgs boson. Based on this discovery, physicists now better understand why some particles, like photons (particles that carry light energy), don't have mass, while others do.

• #### 10. In 1905, with his theory of relativity, Albert Einstein proved that mass and energy are the same thing.

Einstein wrote his most famous equation, E=mc2, called the "mass-energy equivalence equation," to show that the energy (E) stored in an object's matter is equal to its mass (m) multiplied by the speed of light squared (c2)—a gigantic number. This means that a small amount of matter is actually equal to an enormous amount of energy. Confused? Don't worry, Einstein was a pretty smart guy.