Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/47

Book I. difficulty put out of its tate of ret or motion. Upon which account, this Vis insita may, by a mot ignificant name, be called Vis inertiæ or Force of Inactivity. But a body exerts this force only, when another force, impres'd upon it, endeavours to change its condition; and the exercie of this force may be conidered both as reitance and impule; it is reitance in o far as the body, for maintaining its preent tate withtands the force impreed; it is impule in o far as the body, by not eaily giving way to the impres'd force of another, endeavours to change the tate of that other. Reitance is uually acrib'd to bodies at ret, and impule to those in motion: But motion and ret, as commonly conceived, are only relatively ditinguihed; nor are thoe bodies always truly at ret, which commonly are taken to be o.

This force conits in the action only; and remains no longer in the body, when the action is over. For a body maintains every new tate it acquires, by its Vis Inertiæ only. Impres'd forces are of different origines; as from percuion, from preure, from centripetal force.