Principle of vis viva

Vis \Vis\, n.
   1. Force; power.
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   2. (Law)
      (a) Physical force.
      (b) Moral power.
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   {Principle of vis viva} (Mech.), the principle that the
      difference between the aggregate work of the accelerating
      forces of a system and that of the retarding forces is
      equal to one half the vis viva accumulated or lost in the
      system while the work is being done.

   {Vis impressa} [L.] (Mech.), force exerted, as in moving a
      body, or changing the direction of its motion; impressed
      force.

   {Vis inertiae}. [L.]
      (a) The resistance of matter, as when a body at rest is
          set in motion, or a body in motion is brought to rest,
          or has its motion changed, either in direction or in
          velocity.
      (b) Inertness; inactivity.

   Note: Vis intertiae and inertia are not strictly synonymous.
         The former implies the resistance itself which is
         given, while the latter implies merely the property by
         which it is given.

   {Vis mortua} [L.] (Mech.), dead force; force doing no active
      work, but only producing pressure.

   {Vis vitae}, or {Vis vitalis} [L.] (Physiol.), vital force.
      

   {Vis viva} [L.] (Mech.), living force; the force of a body
      moving against resistance, or doing work, in distinction
      from vis mortua, or dead force; the kinetic energy of a
      moving body; the capacity of a moving body to do work by
      reason of its being in motion. See {Kinetic energy}, in
      the Note under {Energy}. The term vis viva is not usually
      understood to include that part of the kinetic energy of
      the body which is due to the vibrations of its molecules.
      [1913 Webster]