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 can change 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 all of them give the same results.

One way is called inertial mass. This tells us how much an object resists being pushed or pulled. We can see this in the formula F = ma, where F is force, m is mass, and a is acceleration.

Another way is gravitational mass. This 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). The way we define the kilogram has changed over time. It was once defined as the mass of a certain amount of water, but this was hard to measure. Later, it was defined as the mass of a special metal object. In 2019, the kilogram was redefined using constants of nature, making it 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.

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.

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, 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.

Newtonian mass

Robert Hooke talked about gravity in 1674. He thought that all celestial bodies pull each other. Isaac Newton learned more about this. He wrote a big book about it in 1687.

Newton showed that gravity pulls objects together. The pull depends on how heavy the objects are and how far apart they are. Bigger objects pull harder. The pull gets weaker when objects are farther away. This explains why planets go around the Sun and why things fall down on Earth.

Newton’s work showed that everything has mass. Mass creates a pull called gravity. This is called universal gravitational mass.

Atomic masses

Main article: Dalton (unit)

We usually measure mass using kilograms, but for very small things like atoms, scientists use a special unit called the dalton. One dalton is one-twelfth the mass of a carbon-12 atom. This makes it easier for scientists to compare the sizes of different atoms.

In relativity

In special relativity, scientists talk about mass in two ways: rest mass and relativistic mass. Rest mass is the amount of mass something has when it is not moving. Relativistic mass changes based on how fast the object is moving. The faster something moves, the more mass it seems to have. This helps scientists see how energy and mass are linked.

In general relativity, mass and gravity are linked. Albert Einstein showed that the force of gravity you feel on Earth is like the force you feel in a moving vehicle. But defining mass in general relativity is more tricky because of how gravity and space-time are connected.

In quantum physics

In classical mechanics, the mass of a particle is a number called m. When we study tiny particles like atoms, we use quantum physics. In this area, the idea of mass changes a little.

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

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