Mass

Mass is a basic idea in science that helps us understand how much stuff is in an object. It is different from weight, which changes depending on where you are, like on Earth or the Moon. Mass stays the same no matter where the object is.

Scientists measure mass in kilograms, and it tells us how much an object will resist moving when a force is applied. For example, a heavy box is harder to push than a light one because it has more mass.

In physics, mass is also linked to gravity. The more mass an object has, the stronger its pull on other objects. This is why planets stay in orbit around the Sun—it has a lot of mass!

Today, we know that even tiny particles have mass, and this mass comes from their connection to a special particle called the Higgs boson. Understanding mass helps scientists explain many things in the universe, from the way stars shine to how atoms are built.

Phenomena

There are different ways to measure mass, and scientists have found that they all give the same results. One way is called inertial mass, which tells us how much an object resists being pushed or pulled. This is shown by the formula F = ma, where F is force, m is mass, and a is acceleration.

Another way is gravitational mass, which tells us how strongly an object can pull on other objects with gravity. Whether we look at how an object pulls on others or how it reacts to others’ pull, the mass we find is the same. This idea is important in the theory of general relativity.

Units of mass

Further information: Orders of magnitude (mass)

The International System of Units (SI) unit of mass is the kilogram (kg). The kilogram is 1000 grams (g). Over time, the way we define the kilogram has changed. Originally, it was defined as the mass of a certain amount of water, but this was hard to measure accurately. Later, it was defined as the mass of a special metal object. In 2019, the kilogram was redefined again using constants of nature, making it even more precise.

Besides the kilogram, there are other units of mass used in science and everyday life. These include the tonne (t), equal to 1000 kg, and the dalton (Da), a tiny unit used for atoms. In some countries, people use the pound (lb) to measure mass. Astronomers often use the solar mass (M_☉), which is the mass of the Sun, to describe the masses of stars and galaxies.

Definition

Mass is a property of objects that tells us how much "stuff" is in them. It helps us understand gravity — the force that pulls objects toward each other. In simple terms, mass is what makes things have weight. For example, a bowling ball has more mass than a tennis ball, so it weighs more.

In everyday life, we often use the words "mass" and "weight" interchangeably. But they are different. Mass stays the same no matter where you are — on Earth, the Moon, or even in space. Weight, however, can change depending on where you are because it depends on gravity. On the Moon, you would weigh less than you do on Earth, but your mass would be exactly the same. This is because the Moon’s gravity is weaker than Earth’s.

Pre-Newtonian concepts

Main article: Weight

Long ago, people thought about how heavy things were by looking at how much stuff they contained. They used the word “weight” to describe both the amount of stuff in an object and how heavy it felt. For example, a goldsmith might say an ounce of gold was just a certain amount of gold, but others felt that weight was about how heavy something felt when held.

Early people noticed that if you had many of the same objects, their total weight was just a certain number times the weight of one object. They used simple tools like balance scales to compare weights. If two objects had the same weight, they also had the same mass, even if one felt heavier because of where it was placed.

Later, scientists like Johannes Kepler studied how planets move around the Sun. He found that planets follow oval paths called ellipses. Around the same time, Galileo Galilei studied how objects fall to the ground. He showed that all objects fall at the same speed, no matter how heavy they are, if you ignore things like air pushing on them. This helped people understand that mass and weight were not exactly the same thing. Finally, Isaac Newton gave mass its own name, separate from weight, in his famous book about how nature works.

Newtonian mass

Robert Hooke published his ideas about gravitational forces in 1674, suggesting that all celestial bodies attract each other. Isaac Newton expanded on these ideas, publishing his major work on the subject in 1687. Newton explained how objects in space are pulled toward each other by gravity.

Newton showed that the pull of gravity depends on both the mass of the objects and the distance between them. He discovered that larger masses pull more strongly, and the pull weakens as objects move farther apart. This helped explain why planets orbit the Sun and why objects fall to the ground on Earth.

Newton’s work introduced the idea that all objects, no matter how big or small, have mass and create a gravitational pull. This became known as universal gravitational mass.

Atomic masses

Main article: Dalton (unit)

We usually measure mass using kilograms, but for tiny things like atoms, scientists use a special unit called the dalton. One dalton is defined as one-twelfth the mass of a carbon-12 atom, which means a carbon-12 atom weighs exactly 12 daltons. This helps scientists compare the sizes of different atoms more easily.

In relativity

In special relativity, scientists describe mass in two ways: rest mass and relativistic mass. Rest mass is the amount of mass an object has when it is not moving. Relativistic mass changes depending on how fast the object is moving. The faster an object moves, the more mass it seems to have. This idea helps scientists understand how energy and mass are connected.

In general relativity, mass and gravity are closely linked. Albert Einstein showed that the force of gravity you feel standing on Earth is similar to the force you feel when you are in an accelerating vehicle. However, defining mass in general relativity is more complex because of the way gravity and space-time are connected.

In quantum physics

In classical mechanics, the mass of a particle appears in certain equations as a number called m. When we look at the tiny particles that make up everything, like atoms and smaller bits, we use something called quantum physics. In this area, the idea of mass changes a bit.

One big idea in quantum physics is that tiny bits of matter can behave like waves. When we use equations to describe these waves, the mass m shows up as a number that helps us understand how these waves move and change. This helps scientists figure out a lot about how very small things act.

There are also ideas about particles that could move faster than light, called tachyons. These are just theories and not things we’ve seen, but they help scientists understand more about how the world works at very tiny sizes. Even with these strange ideas, the rules of physics still make sure that nothing can send information faster than light.