Ve·loc·i·ty n.; pl. Velocities
 1. Quickness of motion; swiftness; speed; celerity; rapidity; as, the velocity of wind; the velocity of a planet or comet in its orbit or course; the velocity of a cannon ball; the velocity of light.
 Note: ☞ In such phrases, velocity is more generally used than celerity.  We apply celerity to animals; as, a horse or an ostrich runs with celerity; but bodies moving in the air or in ethereal space move with greater or less velocity, not celerity. This usage is arbitrary, and perhaps not universal.
 2. Mech. Rate of motion; the relation of motion to time, measured by the number of units of space passed over by a moving body or point in a unit of time, usually the number of feet passed over in a second. See the Note under Speed.
 Angular velocity. See under Angular.
 Initial velocity, the velocity of a moving body at starting; especially, the velocity of a projectile as it leaves the mouth of a firearm from which it is discharged.
 Relative velocity, the velocity with which a body approaches or recedes from another body, whether both are moving or only one.
 Uniform velocity, velocity in which the same number of units of space are described in each successive unit of time.
 Variable velocity, velocity in which the space described varies from instant to instant, either increasing or decreasing; -- in the former case called accelerated velocity, in the latter, retarded velocity; the acceleration or retardation itself being also either uniform or variable.
 Virtual velocity. See under Virtual.
 Note: ☞ In variable velocity, the velocity, strictly, at any given instant, is the rate of motion at that instant, and is expressed by the units of space, which, if the velocity at that instant were continued uniform during a unit of time, would be described in the unit of time; thus, the velocity of a falling body at a given instant is the number of feet which, if the motion which the body has at that instant were continued uniformly for one second, it would pass through in the second. The scientific sense of velocity differs from the popular sense in being applied to all rates of motion, however slow, while the latter implies more or less rapidity or quickness of motion.
 Syn: -- Swiftness; celerity; rapidity; fleetness; speed.
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 Vir·tu·al a.
 1. Having the power of acting or of invisible efficacy without the agency of the material or sensible part; potential; energizing.
    Heat and cold have a virtual transition, without communication of substance.   --Bacon.
 Every kind that lives,
 Fomented by his virtual power, and warmed.   --Milton.
 2. Being in essence or effect, not in fact; as, the virtual presence of a man in his agent or substitute.
    A thing has a virtual existence when it has all the conditions necessary to its actual existence.   --Fleming.
    To mask by slight differences in the manners a virtual identity in the substance.   --De Quincey.
 Principle of virtual velocities Mech., the law that when several forces are in equilibrium, the algebraic sum of their virtual moments is equal to zero.
 Virtual focus Opt., the point from which rays, having been rendered divergent by reflection of refraction, appear to issue; the point at which converging rays would meet if not reflected or refracted before they reach it.
 Virtual image. Optics See under Image.
 Virtual moment (of a force) Mech., the product of the intensity of the force multiplied by the virtual velocity of its point of application; -- sometimes called virtual work.
 Virtual velocity Mech., a minute hypothetical displacement, assumed in analysis to facilitate the investigation of statical problems. With respect to any given force of a number of forces holding a material system in equilibrium, it is the projection, upon the direction of the force, of a line joining its point of application with a new position of that point indefinitely near to the first, to which the point is conceived to have been moved, without disturbing the equilibrium of the system, or the connections of its parts with each other. Strictly speaking, it is not a velocity but a length.
 Virtual work. Mech. See Virtual moment, above.
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