Noun

mass (plural masses)

1. (religion) The Eucharist, now especially in Roman Catholicism.
2. (religion) Celebration of the Eucharist.
3. (religion, usually as the Mass) The sacrament of the Eucharist.
4. A musical setting of parts of the mass.

In physics, mass (from Ancient Greek: μᾶζα) commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: Inertial mass, active gravitational mass and passive gravitational mass. In everyday usage, mass is often taken to mean weight, but in scientific use, they refer to different properties.

The inertial mass of an object determines its acceleration in the presence of an applied force. According to Newton's second law of motion, if a body of mass m is subjected to a force F, its acceleration a is given by F/m.

A body's mass also determines the degree to which it generates or is affected by a gravitational field. If a first body of mass m₁ is placed at a distance r from a second body of mass m₂, each body experiences an attractive force F whose magnitude is

where G is the universal constant of gravitation, equal to 6.67×10 kg m s. This is sometimes referred to as gravitational mass (when a distinction is necessary, M is used to denote the active gravitational mass and m the passive gravitational mass). Repeated experiments since the seventeenth century have demonstrated that inertial and gravitational mass are equivalent; this is entailed in the equivalence principle of general relativity.

Special relativity provides a relationship between the mass of a body and its energy (E = mc). Mass is a conserved quantity. From the viewpoint of any single observer, mass can neither be created or destroyed, and special relativity does not change this understanding. However, relativity adds the fact that all types of energy have an associated mass, and this mass is added to systems when energy is added, and the associated mass is subtracted from systems when the energy leaves. In nuclear reactions, for example, the system does not become less massive until the energy liberated by the reaction is allowed to leave whereby the "missing mass" is carried off with the energy, which itself has mass.

On the surface of the Earth, the weight W of an object is related to its mass m by

where g is the acceleration due to the Earth's gravity, equal to about 9.81 m s. An object's weight depends on its environment, while its mass does not: an object with a mass of 50 kilograms weighs 491 newtons on the surface of the Earth; on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 newtons.