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In physics, mass, more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

1. exerted by an object (active gravitational mass), or

2. experienced by an object (passive gravitational force)

when interacting with a second object. The SI unit of mass is the kilogram (kg).

In everyday usage, mass is often referred to as weight, the units of which are often taken to be kilograms (for instance, a person may state that their weight is 75 kg). In scientific use, however, the term weight refers to a different, yet related, property of matter. Weight is the gravitational force acting on a given body, while mass is an intrinsic property of this body. On the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g is the Earth's gravitational field strength, equal to about 9.81 m s−2. 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.

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 α 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 MA is placed at a distance r from a second body of mass MB, each body experiences an attractive force FG whose magnitude is FG= G MAMB r−2, where G is the universal constant of gravitation, equal to 6.67×10−11 N m2kg-2. This is sometimes referred to as gravitational mass Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are equivalent; since 1915, this observation has been entailed a priori 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. Mass (so long as the type and definition of mass is agreed upon) is a conserved quantity over time. From the viewpoint of any single unaccelerated observer, mass can neither be created or destroyed, and special relativity does not change this understanding (though different observers may not agree on how much mass is present, all agree that the amount does not change over time).

Macroscopically, mass is associated with matter. But on the sub-atomic scale, not only fermions, the particles associated with matter, but also some bosons, the particles that act as force carriers, have rest mass. In the Standard Model of particle physics, mass is described as arising as a consequence of a coupling of the field of which the massive particles are quanta to a postulated additional field, known as the Higgs field.