# Vis inertiae

 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. Inertness; inactivity.

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
Resistance and impetus are different considerations of the same exercise of the vis inertiae. A body, when acted upon, resists actions that would change its state, and in resisting endeavors to change the state of the body that acts upon it.
In Definition 3 of the Principia, Newton very strikingly and paradoxically describes inertia as vis inertiae, the force of inertia, and identifies this force with materiae vis insita, translated here as "inherent force of matter." The Latin insita, used here as a past participle modifying vis, is taken from insero, inserere, insevi, insitus and means implanted, innate, or inborn.
Even though the word "perseverance" (perseverare) ordinarily connotes activity and though in Definition 4 Newton writes that "a body perseveres in any new state solely by the force of inertia," nevertheless, the vis inertiae operates only (solommundo) during a body's change of state in response to an impressed force.
As spoken of by contemporary physicists, inertial or fictitious forces, though related to Newton's vis inertiae, do not have exactly the same meaning, though for both, a body's inertia is acknowledged as a true source of the phenomena under consideration.
(69) In the Opticks, first published in 1704, Newton also makes it clear that the vis inertiae is not an agency within a body that causes it to move or rest: The vis inertiae is a passive principle by which bodies persist in their motion or rest, receive motion in proportion to the force impressing it, and resist as much as they are resisted.
The passivity of the vis inertiae precludes it from being an agency of a body's own uniform rectilinear motion or rest, for that would make it an active principle, which for Newton is a kind of efficient cause.
(77) As such, inertia would be an inherent principle by which a body acts in the characteristic ways that Newton ascribes to the vis inertiae, for everything acts according to its form.
So considered, inertial motion would be consistent with Newton's claim that the vis inertiae of a body does not operate as a continuous mover that keeps a body in uniform rectilinear motion.
(90) Consequently, just as rest was not considered an activity, so too persevering in uniform rectilinear motion was not considered an activity, which is one reason that on Newton's account the vis inertiae is inactive in such motion.
Inertia, considered in terms of form, potency, and act not only better expresses the facts about inertia but also solves an additional problem to which Newton's vis inertiae gives rise.
14; [section] 28, who also speaks of a vis inertiae: "Hae permansiones in eodem statu ...
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