vis inertiae

Inertia \In*er"ti*a\, n. [L., idleness, fr. iners idle. See
   {Inert}.]
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   1. (Physics) That property of matter by which it tends when
      at rest to remain so, and when in motion to continue in
      motion, and in the same straight line or direction, unless
      acted on by some external force; -- sometimes called {vis
      inerti[ae]}. The inertia of a body is proportional to its
      mass.
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   2. Inertness; indisposition to motion, exertion, or action;
      lack of energy; sluggishness.
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            Men . . . have immense irresolution and inertia.
                                                  --Carlyle.
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   3. (Med.) Lack of activity; sluggishness; -- said especially
      of the uterus, when, in labor, its contractions have
      nearly or wholly ceased.
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   {Center of inertia}. (Mech.) See under {Center}.
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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.
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