Barger, Vernon D. Department of Physics, University of Wisconsin–Madison, Madison, Wisconsin.
Last reviewed:January 2021
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The change in the position of an object in a physical space over time. A familiarly encountered phenomenon, motion (see illustration) occurs on all levels of reality, from the constant movements of subatomic particles (conveyed by probabilities of certain positions in quantum mechanics) to superclusters of galaxies; indeed, it can be said that every object in the universe is in motion, relative to every other object. If the position of a material system as measured by a particular observer changes with respect to time, that system is said to be in motion with respect to the observer. The concept of absolute motion, then, has no significance, and only relative motion may be defined, because what one observer measures to be at rest, another observer in a different frame of reference—defined as a base to which to refer physical events occurring at a point in space and time—may regard as being in motion. These and other issues are dealt with in kinematics, which is a branch of mechanics concerning the motion of systems of material particles. See also: Frame of reference; Kinematics; Relative motion; Speed; Time
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