mass

Mass is a property of matter that quantifies the amount of substance in an object and its resistance to changes in motion, known as inertia. In common usage mass is linked to heaviness, but weight—the gravitational force on mass—depends on location and gravity.

The SI unit of mass is the kilogram (kg). Since 2019 the kilogram is defined by fixing the Planck constant, linking mass to fundamental constants through precise electromagnetic measurements. A gram is one-thousandth of a kilogram; the atomic mass unit (u) is used for atomic-scale masses.

Classical physics treats mass as invariant and responsible for inertia. In Einstein’s relativity, rest mass remains constant for a particle, while an object's effective mass can be described as increasing with velocity in some formulations; modern practice emphasizes rest mass and energy instead of relativistic mass.

Mass is measured by comparing with known masses (beam balance) or by observing the motion of bodies under known forces (inertial methods). In high-precision work, mass is tied to fundamental constants. In astronomy, masses are inferred from orbital dynamics, gravitational effects, or luminosity relationships, often expressed in solar masses.

Mass underpins calculations of momentum, gravitational attraction, and energy content (via E=mc^2). It is a conserved quantity in classical processes and a central concept across physics, from subatomic particles to astronomical objects.