Mass

Mass is an important idea in science that helps us understand how things behave. It is a basic property of any object or body. One way scientists think about mass is how strongly an object pulls on other objects because of gravity. This pull depends on how fast we are moving compared to the object.

Earlier, people thought mass was just about how much stuff — or matter — was in an object. But when scientists discovered atoms and tiny particles, they learned that even things made of the same kind of matter can have different masses. Today, mass can also be described as how much an object resists being pushed or pulled, called inertia.

We measure mass in kilograms, the main unit used in science. Mass is not the same as weight, even though we often find out how much mass something has by weighing it. For example, an object will weigh less on the Moon than on Earth because the Moon’s gravity is weaker, but its mass stays the same. In modern physics, the mass of tiny particles comes from their connection to a special particle called the Higgs boson.

Phenomena

There are different ways to measure mass. One way is called inertial mass, which looks at how hard it is to push an object. The harder it is to push, the more inertial mass it has. Another way is active gravitational mass, which measures how strong an object’s pull is on other things. Passive gravitational mass looks at how much an object is pulled by another object’s gravity.

Mass helps us understand how objects move when a force is applied. It also shows how much an object pulls on other objects with gravity. Scientists have found that these different ways of measuring mass give the same results, which is an important idea in physics.

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). It was first defined in 1795 as the mass of one cubic decimetre of water at the melting point of ice. Later, it was changed to the mass of special metal objects. But these objects changed tiny amounts over time.

In 2019, the kilogram was redefined using constants of nature, like the speed of light and the Planck constant. Other units used with SI units include the tonne (t), equal to 1000 kg, and the dalton (Da), about 1/12 the mass of a free carbon-12 atom. Outside SI, units include the slug (sl), the pound (lb), the Planck mass, and the solar mass (M_☉), the mass of the Sun.

Definition

Mass is a special property of an object that tells us how much "stuff" is in it. In simple terms, it helps us understand why objects pull toward each other due to gravity. Imagine you have a heavy ball and a light ball. The heavy ball has more mass because it has more material in it.

In everyday life, we often use the words "mass" and "weight" interchangeably. For example, we might say a person weighs 75 kilograms. But scientists make a clear difference between the two. Mass is a property of the object itself and never changes. Weight, however, can change depending on where you are. For example, you would weigh slightly less on the Moon than on Earth, even though your mass stays the same. This is because the Moon’s gravity is weaker than Earth’s.

Pre-Newtonian concepts

Main article: Weight

Depiction of early balance scales in the Papyrus of Hunefer (dated to the 19th dynasty, c. 1285 BCE). The scene shows Anubis weighing the heart of Hunefer.

Long ago, people thought about how heavy things were. They called this "weight," but it was really about how much stuff, or mass, was in an object. For example, a goldsmith might say a piece of gold weighed an ounce, meaning it had a certain amount of gold.

Early people noticed that if you have more of the same objects, they weigh more. Imagine having one apple versus ten apples—the ten apples weigh more because there are more of them. They used balance scales to compare weights. If two objects balance, they have the same mass, even if they feel different when lifted because of where they are.

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 their mass, if you ignore things like air pushing on them. This helped people understand that mass and weight are related but not the same thing.

Newtonian mass

Isaac Newton, 1689

Robert Hooke shared ideas about gravity in 1674, suggesting that objects pull toward each other. Isaac Newton later built on these ideas, explaining how gravity works between any two objects. He showed that the force of gravity depends on the mass of the objects and the distance between them.

Newton’s work helped us understand mass in two ways: how it affects gravity and how it resists movement. By studying how objects move and pull each other, scientists can measure mass and learn more about how our universe works.

Earth's Moon		Mass of Earth
Semi-major axis	Sidereal orbital period	Mass of Earth
0.002 569 AU	0.074 802 sidereal year	1.2 π 2 ⋅ 10 − 5 AU 3 y 2 = 3.986 ⋅ 10 14 m 3 s 2 {\displaystyle 1.2\pi ^{2}\cdot 10^{-5}{\frac {{\text{AU}}^{3}}{{\text{y}}^{2}}}=3.986\cdot 10^{14}{\frac {{\text{m}}^{3}}{{\text{s}}^{2}}}}
Earth's gravity	Earth's radius
9.806 65 m/s²	6 375 km

Atomic masses

Main article: Dalton (unit)

We usually measure the mass of objects using kilograms. But when looking at very tiny things like atoms, scientists use a different unit called the dalton. One dalton is defined as one-twelfth the mass of a carbon-12 atom. This means a carbon-12 atom has a mass of exactly 12 daltons. This unit helps scientists compare the sizes of different atoms more easily.

In relativity

In special relativity, scientists look at mass in new ways. They talk about two kinds: rest mass and relativistic mass. Rest mass is what we usually think of as mass — it stays the same no matter how fast something is moving. Relativistic mass changes with speed; it gets bigger the faster an object moves.

These ideas show that mass and energy are really two sides of the same coin. When something moves or changes, its mass and energy change together. Even when ice melts into water, the tiny amount of heat added makes the water a little heavier! This is because energy has mass, though the amount is very small.

In quantum physics

In classical mechanics, mass is a key part of equations that describe how objects move. When we look at the tiny particles that make up everything around us, mass still plays an important role. In quantum physics, mass shows up in special math equations that help scientists understand how these tiny particles behave.

One important idea is that mass helps connect to a field called the Higgs field. This connection gives particles their mass. Even though we can't see this field directly, it is believed to be everywhere and gives particles the property we call mass.

Some theories talk about particles that could move faster than light, called tachyons. These are still just ideas and not proven to exist. Even in these ideas, the rules of physics make sure that nothing can break the important speed limit of light. These theories help scientists understand more about how the universe works and how particles stay stable.