Mass
The mass of a body is its inertia or resistance to change of motion. More precisely, it is a property of the body that determines the body's acceleration under the influence of a given force. Mass can therefore be measured either by the amount of force necessary to impart to the body a given motion in a given time or by the acceleration produced by a given force.
The absolute metric unit of mass is the gram, which is the mass of a body whose velocity increases by one centimeter per second each second if acted upon by a force of one dyne. Other common units are the kilogram (1,000 grams) and the pound (453.592 grams). For velocities that are small as compared with the speed of light, the mass of a body is a constant, characteristic of the body and independent of its location—in contrast to weight, which varies with the body's place on Earth or in the universe.
Although fundamental to science and, together with length and time, the basis of all measurements in physics, the concept of mass was unambiguously defined only at the end of the nineteenth century. However, its rudimentary sources, systematically employed long before by Isaac Newton and to some extent already by Johannes Kepler, can be traced back to early Neoplatonic ideas concerning the inactivity of matter as opposed to the spontaneity of mind.
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