For other situations, such as when objects are subjected to mechanical accelerations from forces other than the resistance of a planetary surface, the weight force is proportional to the mass of an object multiplied by the total acceleration away from free fall, which is called the proper acceleration. 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 = – ma, where a is the proper acceleration of the object caused by all influences other than gravity. (Again, if gravity is the only influence, such as occurs when an object falls freely, its weight will be zero).

The International System of Units (SI) unit of mass is the kilogram (kg). The kilogram is 1000grams (g), and was first defined in 1795 as the mass of one cubic decimetre of water at the melting point of ice. However, because precise measurement of a cubic decimetre of water at the specified temperature and pressure was difficult, in 1889 the kilogram was redefined as the mass of a metal object, and thus became independent of the metre and the properties of water, this being a copper prototype of the grave in 1793, the platinum Kilogramme des Archives in 1799, and the platinum-iridium International Prototype of the Kilogram (IPK) in 1889. There are several distinct phenomena that can be used to measure mass. Although some theorists have speculated that some of these phenomena could be independent of each other, [2] current experiments have found no difference in results regardless of how it is measured: 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.the electronvolt (eV), a unit of energy, used to express mass in units of eV/ c 2 through mass–energy equivalence Inertial mass measures an object's resistance to being accelerated by a force (represented by the relationship F = ma). where W is the weight of the collection of similar objects and n is the number of objects in the collection. Proportionality, by definition, implies that two values have a constant ratio:

On 25 August 1609, Galileo Galilei demonstrated his first telescope to a group of Venetian merchants, and in early January 1610, Galileo observed four dim objects near Jupiter, which he mistook for stars. However, after a few days of observation, Galileo realized that these "stars" were in fact orbiting Jupiter. These four objects (later named the Galilean moons in honor of their discoverer) were the first celestial bodies observed to orbit something other than the Earth or Sun. Galileo continued to observe these moons over the next eighteen months, and by the middle of 1611, he had obtained remarkably accurate estimates for their periods. Humans, at some early era, realized that the weight of a collection of similar objects was directly proportional to the number of objects in the collection: Donnerstein, Edward. "Mass Media, General View." Encyclopedia of Violence, Peace, & Conflict (Second Edition). Ed. Kurtz, Lester. Oxford: Academic Press, 2008. 1184-92. Print. The force is said to be a natural existence or phenomenon that can cause a change in the motion or rest state of a body. Moreover, the force applied in the form of stress may cause a change in the dimension of the object. Hooke’s Law explained the principle of stress. According to this law, the stress imposed on a body will be directly proportional to the strain causing that body. Hooke’s law postulated the spring’s constant, in which the spring length is increased as much as the force is applied to stretch it. Therefore, the spring constant is also called the force constant. Force

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the mass of a particle, as identified with its inverse Compton wavelength ( 1cm −1 ≘ 3.52 ×10 −41kg)