Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. Unlike weight, the mass of something at rest stays the same regardless of location. Mass is a central concept of classical mechanics and related subjects, and there are several forms of mass within the framework of relativistic kinematics.
Units of mass
In the SI system of units, mass is measured in kilograms (kg). Many other units of mass are also employed, such as: grams (g), tonne, pounds, ounces, long and short tons, slugs etc.
The eV/c2 unit is based on the electron volt (eV), which is normally used as a unit of energy. However, because of the relativistic connection between (rest) mass and energy, E = mc2, it is possible to use any unit of energy as a unit of mass instead. Thus, in particle physics where mass and energy are often interchanged, it is common to use not only eV/c2 but even simply eV as a unit of mass (roughly 1.783 × 10-36 kg). Masses are sometimes also expressed in terms of inverse lengths. Here one identifies the mass of a particle with its inverse Compton wavelength ( kg).
Because the gravitational acceleration (g) is approximately constant on the surface of the earth, and also because mass-balances do not depend on the local value of g, a unit like the pound is often used to measure either mass or force. (e.g. weight). When the pound is used as a measure of mass (where g does not enter in), it is officially in the English system defined in terms of the kg, as 1 lb = 0.453 592 37 kg (see force.) In this case the English system unit of force is the poundal. By contrast, when the pound is used as the unit of force, the English unit of mass is the slug (mass).
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