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In physics, mass is a property of a physical body which determines the strength of its mutual gravitational attraction to other bodies, its resistance to being accelerated by a force, and in the theory of relativity gives the mass–energy content of a system. The SI unit of mass is the kilogram (kg). Mass is not the same as weight, even though we often calculate an object's mass by measuring its weight with a spring scale instead of comparing it to known masses. An object on the Moon would weigh less than it would on Earth because of the lower gravity, but it would still have the same mass. For everyday objects and energies well-described by Newtonian physics, mass describes the amount of matter in an object. However, at very high speeds or for subatomic particles, special relativity shows that energy is an additional source of mass. Thus, any stationary body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction. There are several distinct phenomena which can be used to measure mass. Although some theorists have speculated some of these phenomena could be independent of each other, current experiments have found no difference among any of the ways used to measure mass: * ''Inertial mass'' measures an object's resistance to being accelerated by a force (represented by the relationship ). * ''Active gravitational mass'' measures the gravitational force exerted by an object. * ''Passive gravitational mass'' measures the gravitational force experienced by an object in a known gravitational field. * ''Mass–energy'' measures the total amount of energy contained within a body, using . The mass of an object determines its acceleration in the presence of an applied force. This phenomenon is called inertia. According to Newton's second law of motion, if a body of fixed mass ''m'' is subjected to a single 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 ''m''A is placed at a distance ''r'' (center of mass to center of mass) from a second body of mass ''m''B, each body experiences an attractive force , where is the "universal gravitational constant". 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 identical; since 1915, this observation has been entailed ''a priori'' in the equivalence principle of general relativity. == Units of mass == The standard International System of Units (SI) unit of mass is the kilogram (kg). The kilogram is 1000 grams (g), first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the international prototype kilogram, and as such is independent of the meter, or the properties of water. As of January 2013, there are several proposals for redefining the kilogram yet again, including a proposal for defining it in terms of the Planck constant. Other units are accepted for use in SI: * the tonne (t) (or "metric ton") is equal to 1000 kg. * the electronvolt (eV) is a unit of energy, but because of the mass–energy equivalence it can easily be converted to a unit of mass, and is often used like one. In this context, the mass has units of eV/''c''2. The electronvolt is common in particle physics. * the atomic mass unit (u) is 1/12 of the mass of a carbon-12 atom, approximately .〔Since the Avogadro constant ''N''A is defined as the number of atoms in 12 g of carbon-12, it follows that 1 u is exactly 1/(103''N''A) kg.〕 The atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units include: * the slug (sl) is an Imperial unit of mass (about 14.6 kg) similar to the kilogram. * the pound (lb) is a unit of both mass and force, used mainly in the United States (about 0.45 kg or 4.5 N). In scientific contexts where pound (force) and pound (mass) need to be distinguished, SI units are usually used instead. * the Planck mass (''m''P) is the maximum mass of point particles (about ). It is used in particle physics. * the solar mass () is defined as the mass of the sun. It is primarily used in astronomy to compare large masses such as stars or galaxies (≈). * the mass of a very small particle may be identified with its inverse Compton wavelength (). * the mass of a very large star or black hole may be identified with its Schwarzschild radius (). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「mass」の詳細全文を読む スポンサード リンク
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