Mass

Mass is a measure of how much matter a thing contains. More precisely, it quantifies a body’s resistance to acceleration when a force is applied — what Newton called inertia. Push two objects with the same force: the one that accelerates less has more mass.

Mass also determines gravitational attraction. Every object with mass pulls on every other object with mass. The more mass, the stronger the pull. This is why mass and weight are easy to confuse — weight is the force of gravity acting on a mass, so it changes with location (you weigh less on the Moon), but mass stays the same everywhere.

In physics, mass is measured in kilograms. In Einstein’s theory of relativity, mass and energy are interconvertible: $E = mc^2$ says that a small amount of mass corresponds to an enormous amount of energy. This means mass is not just “stuff” — it is a form of concentrated energy, and the distinction between the two blurs at fundamental scales.

Mass is one of the defining qualities of an object. A thing without mass — like a photon — can exist and carry energy, but it cannot be weighed, cannot resist being pushed, and travels at the speed of light. Mass is what makes things sluggish, heavy, and gravitationally present.