relative velocity

From The Collaborative International Dictionary of English v.0.48:

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].]
   [1913 Webster]
   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.
      [1913 Webster]

   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.
         [1913 Webster]

   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
      [1913 Webster]

   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

   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.
      [1913 Webster]

   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.
         [1913 Webster]

   Syn: Swiftness; celerity; rapidity; fleetness; speed.
        [1913 Webster]
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