mass

=Mass=

toc Read here about a physical property that can be measured and depicted in a scalometer.

=Overview=

The concept of mass is one of the most fundamental notions in physics, comparable in importance only to those of space and time, and just as difficult to define.

In physics, mass refers to any of following three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass, and passive gravitational mass.

Mass must be distinguished from matter in physics, because 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 their weight is 75 kg). In proper scientific use, however, the two terms refer to different, yet related, properties of matter.

Einstein's equation E=MC^2 adds the fact that all types of energy have an associated mass, and this mass is added to systems when energy is added, and the associated mass is subtracted from systems when the energy leaves. In such cases, the energy leaving or entering the system carries the added or missing mass with it, since this energy itself has mass. Thus, mass remains conserved when the location of all mass is taken into account.

Force is related to mass when considering the weight of an object on Earth, according to the formula 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.

Summary of mass related phenomena
In physical science, one may distinguish conceptually between at least seven attributes of mass, or seven physical phenomena that can be explained using the concept of mass:[1] Inertial mass, gravitational mass, and the various other mass-related phenomena are conceptually distinct. However, every experiment to date has shown these values to be proportional, and this proportionality gives rise to the abstract concept of mass. If, in some future experiment, one of the mass-related phenomena is shown to not be proportional to the others, then that specific phenomena will no longer be considered a part of the abstract concept of mass.
 * The amount of matter in certain types of samples can be exactly determined through electrodeposition[disambiguation needed][clarification needed] or other precise processes. The mass of an exact sample is determined in part by the number and type of atoms or molecules it contains, and in part by the energy involved in binding it together (which contributes a negative "missing mass," or mass deficit).
 * Inertial mass is a measure of an object's resistance to changing its state of motion 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 is a measure of the strength of an object’s gravitational flux (gravitational flux is equal to the surface integral of gravitational field over an enclosing surface). Gravitational field can be measured by allowing a small ‘test object’ to freely fall and measuring its free-fall acceleration. For example, an object in free-fall near the Moon will experience less gravitational field, and hence accelerate slower 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–energy equivalence. 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 through which measurable amounts of mass and energy are converted into each other. 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. Curvature is extremely weak and difficult to measure. For this reason, curvature wasn’t 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) than 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.

=Units of Mass= In the International System of Units (SI), mass is measured in kilograms (kg). Other units are accepted for use in SI:
 * The tonne (t) is equal to 1000 kg.
 * The electronvolt (eV) is primarily a unit of energy, but because of the mass-energy equivalence it can also function as a unit of mass. In this context it is denoted eV/c2, or simply as eV. The electronvolt is common in particle physics.
 * The atomic mass unit (u) is defined so that a single carbon-12 atom has a mass of 12 u; 1 u is approximately 1.66×10^−27 kg, 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/(10^3 N(A)) kg. The atomic mass unit is convenient for expressing the masses of atoms and molecules.

The kilogram is an SI base unit. It is defined as the mass of the international prototype of the kilogram.

Outside the SI system, a variety of different mass units are used, depending on context, such as the slug (sl), the pound (lb), the Planck mass (mP), and the solar mass (M).

In normal situations, the weight of an object is proportional to its mass, which usually makes it unproblematic to use the same unit for both concepts. However, the distinction between mass and weight becomes important for measurements with a precision better than a few percent (because of slight differences in the strength of the Earth's gravitational field at different places), and for places far from the surface of the Earth, such as in space or on other planets.

A mass can sometimes be expressed in terms of length. The mass of a very small particle may be identified with its inverse Compton wavelength (1 cm−1 ≈ 3.52×10−41 kg). The mass of a very large star or black hole may be identified with its Schwarzschild radius (1 cm ≈ 6.73×1024 kg). The mass is the electric dipole moment.

The maximum mass is the sum of all mass in the universe.

The minimum mass approaches zero: a photon of light has zero mass, though its energy can be converted to mass via E=MC^2.

=Jammer on Mass=

Max Jammer published a definitive history of the concept of mass. His book begins with an analysis of the persistent difficulties of defining inertial mass in a noncircular manner and discusses the related question of whether mass is an observational or a theoretical concept. It then studies the notion of mass in special relativity and the delicate problem of whether the relativistic rest mass is the only legitimate notion of mass and whether it is identical with the classical (Newtonian) mass. This is followed by a critical analysis of the different derivations of the famous mass-energy relationship E = mc^2 and its conflicting interpretations. Jammer then devotes a chapter to the distinction between inertial and gravitational mass and to the various versions of the so-called equivalence principle with which Newton initiated his Principia but which also became the starting point of Einstein's general relativity, which supersedes Newtonian physics. The book concludes with a presentation of recently proposed global and local dynamical theories of the origin and nature of mass.

=Scale=

Problems with scaling mass relate to structure, being the definition of how the mass is understood as cohering in one whole. It is such concepts that enable us to disaggregate mass, such as differentiating the mass of one person from another's, and the mass of one person's body from the mass of planet Earth as a whole.