Why Opposite Movements Annihilate Each Other and Renew Themselves in Elastic and Resilient Bodies

Abstract

From what was said above it is clear that two bodies colliding in opposite movements sometimes lose their impetus completely and are reduced to immobility, sometimes their velocities remain absolutely unaltered and of the same vigour. But it seems impossible that the impulses, after being weakened or extinguished in the collision of the bodies, revivify afterwards since one and the same cause, namely the repulsion of an opposite movement, cannot generate immobility and movement in the same subject. But it is obvious that this occurs in elastic and resilient bodies such as are machines and other bodies of the same nature. Indeed, a playing ball is at first slowed down by an elastic net and then very impetuously propelled. A ball inflated with air or a water-skin, like other machines which are constricted and dilated, is also at first slowed down and afterwards very impetuously impelled. In all of these examples one of the two bodies is immobile. It is either the ball or the racket.