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MassThis article is about physics. For the sense used in religion, see Mass (liturgy). For the Mass as a type of classical music composition, see Mass (music).
Mass is a property of physical objects which, roughly speaking, measure the amount of matter contained in an object. It is a central concept of classical mechanics and related subjects. In the SI system of measurement, mass is measured in kilograms. Strictly speaking, mass refers to two quantities:
Inertial MassInertial mass is determined using Newton's second and third laws of motion (see classical mechanics.) Given an object with a known inertial mass, we can obtain the inertial mass of any other object by making the two objects exert a force on each other. According to Newton's third law, the forces experienced by each object will have equal magnitude. This allows us to study how the two objects resist similar applied forces. Suppose we have two objects, A and B, with inertial masses m_{A} (which is known) and m_{B} (which we wish to determine.) We will assume these masses to be constant. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on A by B, which we denote F_{AB}, and the force exerted on B by A, which we denote F_{BA}. According to Newton's second law,
Newton's third law states that the two forces are equal and opposite, i.e.
In the above discussion, we assumed that the masses of A and B are constant. This is a fundamental assumption, known as the conservation of mass, and is based on the expectation that matter can never be created or destroyed, only split up or recombined. (The implications of special relativity are discussed below.) It is sometimes useful to treat the mass of an object as changing with time: for example, the mass of a rocket decreases as the rocket fires. However, this is an approximation based on ignoring pieces of matter which enter or leave the system. In the case of the rocket, these pieces correspond to the ejected propellent; if we were to measure the total mass of the rocket and its propellent, we would find that it is conserved. Gravitational MassConsider two objects A and B with gravitational masses M_{A} and M_{B}, at a distance of |r_{AB}| apart. Newton's law of gravitation states that the magnitude of the gravitational force which each object exerts on the other is
Equivalence of Inertial and Gravitational MassesExperiments have found inertial and gravitational mass to be equal, to a high level of precision. These experiments are essentially tests of the well-known phenomenon, first observed by Galileo, that objects fall at a rate irrespective of their masses (in the absence of factors such as friction.) Suppose we have an object with inertial and gravitational masses m and M respectively. If gravity is the only force acting on the object, the combination of Newton's second law and gravitational law gives its acceleration a as
Consequences of RelativityIn the special theory of relativity, "mass" refers to the inertial mass of an object as measured in the frame of reference in which it is at rest (which is known as its "rest frame".) The above method for determining inertial masses remains valid, provided we ensure that the speed of the object is always much smaller than the speed of light, so that classical mechanics is valid.
Historically, the term "mass" was used for the quantity E/c². This was called the "relativistic mass", and m called the "rest mass". This terminology is now discouraged by physicists, because there is no need for two terms for the energy of a particle, and because it creates confusion when speaking of "massless" particles. In this article, we will always mean the rest mass whenever we refer to "mass". For more details, see the Usenet Relativity FAQ in the External Links.
In relativistic mechanics, the mass of a free particle is related to its energy and momentum by the following equation:
This equation governs the mechanics of massless particles such as photons, the particles of light.
See alsoExternal link
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