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In physics, mass (from Ancient Greek: μᾶζα) commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent:

Mass must be distinguished from matter in physics, however, since matter is a poorly-defined concept, and although all types of agreed-upon matter exhibit mass, it is also the case that many types of energy which are not matter— such as potential energy, kinetic energy, and trapped electromagnetic radiation (photons)— also exhibit mass. Thus, all matter has the property of mass, but not all mass is associated with identifiable matter.

In everyday usage, mass is often used interchangeably with weight, and the units of weight are often taken to be kilograms (for instance, a person may state that his weight is 75 kg). In proper scientific use, however, the two terms refer to different, yet related, properties of matter.

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 fixed 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 6.67×10−11
 kg−1 m3 s−2
. 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 17th century have demonstrated that inertial and gravitational mass are equivalent; this is entailed in the equivalence principle of general relativity.

Special relativity shows that rest mass (or invariant mass) and rest energy are essentially equivalent, via the well-known relationship (E = mc2). This same equation also connects relativistic mass and "relativistic energy" (total system energy). These are concepts that are related to their "rest" counterparts, but they do not have the same value, in systems where there is a net momentum. In order to deduce any of these four quantities from any of the others, in any system which has a net momentum, an equation that takes momentum into account is needed.

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