Mass

Mass (also called cufnaan) is a fundamental property that all matter possesses. It measures the amount of matter an object contains and how much it resists changes in its motion (inertia), as well as the strength of its gravitational interaction. The standard unit of mass in the international SI system is the kilogram (kg).

Mass must be distinguished from weight. Weight is the force that gravity exerts on an object and changes depending on location—for example, an object from Earth transported to the Moon will have a different weight. Its mass remains constant, but its weight decreases.

Types of mass

Physics distinguishes two aspects of mass. Inertial mass appears in Newton's second law: the force applied to an object equals its mass multiplied by its acceleration (F = ma). Gravitational mass determines the force of gravity between two objects. Numerous experiments have shown that these two values are identical to very high precision; this is the basis of the equivalence principle in Einstein's general theory of relativity.

Mass and energy

Einstein's special theory of relativity connects mass and energy through the equation E = mc², where c is the speed of light. Therefore, when a system's energy increases, its mass also increases. In nuclear fission, some mass is converted into released energy; this is the source of nuclear reactor power and stellar energy.

Measurement

Mass is measured using a balance scale, which compares an object to a known standard mass, or a digital device that measures gravitational force. The kilogram was historically defined by a standard platinum cylinder kept in France; in 2019 it was redefined in terms of Planck's constant, so that the unit would be based on a natural constant rather than a specific artifact.