action



action defined in 1939 year

action - Action (Lat. aclio);
action - Term used in various senses. In physical science action at a distance is referred to in Newton's third law of motion thus: "to every action there is always an equal and contrary reaction; or the mutual actions of any two bodies are always equal and oppositely directed." When one body acts on another body so as to influence it, then the whole phenomenon of the mutual action is called a stress. Hence Newton's third law states that all forces are of the nature of a stress between bodies or portions of matter, since every force must be accompanied by an equal and oppositely directed reaction.

If you press your hand on a table, you feel the table pressing your hand. If a horse is towing a boat, the forward pull of the horse on the towrope is exactly equal to the backward pull of the towrope on the horse. In such a case it is easy to see how the horse exerts a force on the boat, because it is by the stretched rope. But sometimes it is not possible to detect the agency by which one body acts on another—as when a piece of iron is attracted by a magnet, or the waters of the earth are attracted by the moon. In these instances we ordinarily use language which implies that matter can act upon matter at a distance, without any connecting agency, by means of an agent which we call force. Nevertheless, we are unable to think of one portion of matter acting on another without something connecting the two, by which the potion is transmitted. This something was called the medium of the transmission of these forces, and although no one could imagine the real nature of this medium which enabled bodies to attract one another even at stellar distances, jet it was confidently asserted that such a medium, usually called the luminiferous or light-bearing ether, did exist. The famous Michelson-Morley experiments between 1887 and 1904 indicated that the ether of space was useless as a postulated medium for the transmission of action between bodies; and, further the development of the theory of electromagnetic radiation to include light, heat and all-forms of radiation, made the conception of a physical ether increasingly untenable. In 1905 Einstein developed his theory of relativity, which made the ether an entirely superfluous concept. No explanation of action at a distance has been provided, and the propagation of radiant energy remains unex la ned. See Ether; Light; Radiation.

near action in Knolik


actinozoahome
letter "A"
start from "AC"
action francaise, l'

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