Virtual velocity

Velocity \Ve*loc"i*ty\, n.; pl. {Velocities}. [L. velocitas,
   from velox, -ocis, swift, quick; perhaps akin to volare to
   fly (see {Volatile}): cf. F. v['e]locit['e].]
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   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.
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   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.
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   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}.
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   {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}.
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   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.
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   Syn: Swiftness; celerity; rapidity; fleetness; speed.
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Virtual \Vir"tu*al\ (?; 135), a. [Cf. F. virtuel. See {Virtue}.]
   1. Having the power of acting or of invisible efficacy
      without the agency of the material or sensible part;
      potential; energizing.
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            Heat and cold have a virtual transition, without
            communication of substance.           --Bacon.
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            Every kind that lives,
            Fomented by his virtual power, and warmed. --Milton.
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   2. Being in essence or effect, not in fact; as, the virtual
      presence of a man in his agent or substitute.
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            A thing has a virtual existence when it has all the
            conditions necessary to its actual existence.
                                                  --Fleming.
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            To mask by slight differences in the manners a
            virtual identity in the substance.    --De Quincey.
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   {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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