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Mass - Wikipedia. This article is about the scientific concept. For the substance of which all physical objects consist, see Matter. For other uses, see Mass (disambiguation). In physics, mass is a property of a physical body.
It is the measure of an object's resistance to acceleration (a change in its state of motion) when a net force is applied. The basic SI unit of mass is the kilogram (kg). Mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force. In Newtonian physics, mass can be generalized as the amount of matter in an object.
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However, at very high speeds, special relativity postulates 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. Although some theorists have speculated that some of these phenomena could be independent of each other. The inertia and the inertial mass describe the same properties of physical bodies at the qualitative and quantitative level respectively, by other words, the mass quantitatively describes the 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 is subject to an attractive force Fg = Gm. Am. B/r. 2, where G = 6. This is sometimes referred to as gravitational mass.
The kilogram is 1. Then in 1. 88. 9, 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 2. 01. Planck constant. In this context, the mass has units of e.
V/c. 2 (where c is the speed of light). The electronvolt and its multiples, such as the Me. V (megaelectronvolt), are commonly used in particle physics. In scientific contexts where pound (force) and pound (mass) need to be distinguished, SI units are usually used instead. Planck mass (m. P) is the maximum mass of point particles (about 6. It is used in particle physics. M. It is primarily used in astronomy to compare large masses such as stars or galaxies (.
Every massive object is believed to exhibit all five properties. However, due to extremely large or extremely small constants, it is generally impossible to verify more than two or three properties for any object. There are a number of ways mass can be measured or operationally defined: Inertial mass is a measure of an object's resistance to acceleration when a force is applied. It is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says the body of greater mass has greater inertia.
Active gravitational mass. Gravitational field can be measured by allowing a small . For example, an object in free fall near the Moon is subject to a smaller gravitational field, and hence accelerates more slowly, than the same object would if it were in free fall near the Earth. The gravitational field near the Moon is weaker because the Moon has less active gravitational mass.
Passive gravitational mass is a measure of the strength of an object's interaction with a gravitational field. Passive gravitational mass is determined by dividing an object's weight by its free- fall acceleration. Two objects within the same gravitational field will experience the same acceleration; however, the object with a smaller passive gravitational mass will experience a smaller force (less weight) than the object with a larger passive gravitational mass. Energy also has mass according to the principle of mass.
This equivalence is exemplified in a large number of physical processes including pair production, nuclear fusion, and the gravitational bending of light. Pair production and nuclear fusion are processes in which measurable amounts of mass are converted to energy, or vice versa. In the gravitational bending of light, photons of pure energy are shown to exhibit a behavior similar to passive gravitational mass.
Curvature of spacetime is a relativistic manifestation of the existence of mass. Such curvature is extremely weak and difficult to measure. For this reason, curvature was not discovered until after it was predicted by Einstein's theory of general relativity.
Extremely precise atomic clocks on the surface of the Earth, for example, are found to measure less time (run slower) when compared to similar clocks in space. This difference in elapsed time is a form of curvature called gravitational time dilation. Other forms of curvature have been measured using the Gravity Probe B satellite. Quantum mass manifests itself as a difference between an object's quantum frequency and its wave number. The quantum mass of an electron, the Compton wavelength, can be determined through various forms of spectroscopy and is closely related to the Rydberg constant, the Bohr radius, and the classical electron radius.
The quantum mass of larger objects can be directly measured using a Watt balance. In relativistic quantum mechanics, mass is one of the irreducible representation labels of the Poincar. For instance, a person's weight may be stated as 7. In a constant gravitational field, the weight of an object is proportional to its mass, and it is unproblematic to use the same unit for both concepts. But because of slight differences in the strength of the Earth's gravitational field at different places, the distinction becomes important for measurements with a precision better than a few percent, and for places far from the surface of the Earth, such as in space or on other planets.
No matter how strong the gravitational field, objects in free fall are weightless, though they still have mass. For example, when a body is at rest in a gravitational field (rather than in free fall), it must be accelerated by a force from a scale or the surface of a planetary body such as the Earth or the Moon. This force keeps the object from going into free fall. Weight is the opposing force in such circumstances, and is thus determined by the acceleration of free fall. On the surface of the Earth, for example, an object with a mass of 5. By contrast, on the surface of the Moon, the same object still has a mass of 5.
Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g = 7. Through such mechanisms, objects in elevators, vehicles, centrifuges, and the like, may experience weight forces many times those caused by resistance to the effects of gravity on objects, resulting from planetary surfaces.
In such cases, the generalized equation for weight W of an object is related to its mass m by the equation W = . On the subatomic scale, not only fermions, the particles often associated with matter, but also some bosons, the particles that act as force carriers, have rest mass. Another problem for easy definition is that much of the rest mass of ordinary matter derives from the binding energy (potential energy) holding their quarks together and other forms of energy rather than the sum of the rest masses of the individual particle constituents. For example, only 1% of the rest mass of matter is accounted for by the rest mass of its elementary quarks and electrons. From a fundamental physics perspective, mass is the number describing under which the representation of the little group of the Poincar.
In the Standard Model of particle physics, this symmetry is described as arising as a consequence of a coupling of particles with rest mass to a postulated additional field, known as the Higgs field. The total mass of the observable universe is estimated at between 1. In classical mechanics, Newton's third law implies that active and passive gravitational mass must always be identical (or at least proportional), but the classical theory offers no compelling reason why the gravitational mass has to equal the inertial mass. That it does is merely an empirical fact. Albert Einstein developed his general theory of relativity starting from the assumption that this correspondence between inertial and (passive) gravitational mass is not accidental: that no experiment will ever detect a difference between them (the weak version of the equivalence principle).
However, in the resulting theory, gravitation is not a force and thus not subject to Newton's third law, so . The most important consequence of this equivalence principle applies to freely falling objects. Suppose we have an object with inertial and gravitational masses m and M, respectively. If the only force acting on the object comes from a gravitational field g, combining Newton's second law and the gravitational law yields the accelerationa=Mmg. This phenomenon is referred to as the .
It is commonly stated that Galileo obtained his results by dropping objects from the Leaning Tower of Pisa, but this is most likely apocryphal; actually, he performed his experiments with balls rolling down nearly frictionless inclined planes to slow the motion and increase the timing accuracy. Increasingly precise experiments have been performed, such as those performed by Lor. More precise experimental efforts are still being carried out. The universality of free- fall only applies to systems in which gravity is the only acting force.
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