difficulty put out of its ſtate of reſt or motion. Upon which account, this Vis insita may, by a moſt ſignificant name, be called Vis inertiæ or Force of Inactivity. But a body exerts this force only, when another force, impreſs'd upon it, endeavours to change its condition; and the exerciſe of this force may be conſidered both as reſiſtance and impulſe; it is reſiſtance in ſo far as the body, for maintaining its preſent ſtate withſtands the force impreſſed; it is impulſe in ſo far as the body, by not eaſily giving way to the impreſs'd force of another, endeavours to change the ſtate of that other. Reſiſtance is uſually aſcrib'd to bodies at reſt, and impulſe to those in motion: But motion and reſt, as commonly conceived, are only relatively diſtinguiſhed; nor are thoſe bodies always truly at reſt, which commonly are taken to be ſo.
Definition IV.
This force conſiſts in the action only; and remains no longer in the body, when the action is over. For a body maintains every new ſtate it acquires, by its Vis Inertiæ only. Impreſs'd forces are of different origines; as from percuſſion, from preſſure, from centripetal force.
