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principle of vis viva
From The Collaborative International Dictionary of English v.0.48:
Vis \Vis\, n. 1. Force; power. [1913 Webster] 2. (Law) (a) Physical force. (b) Moral power. [1913 Webster] 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]