On the Force of Percussion

DE VI PERCUSSIONIS. CAP. I.

summary is nothing other than the measure or degree of its velocity, and this kind of force is usually called impetus.

It must be observed, however, that the transition of local motion can be twofold, namely, either of the whole body or of a part of it. The transition or motion of the whole body certainly occurs when no part of the same body can be assigned that does not change its place. If, however, with one part of the same body resting, the remaining parts revolve around it as if around a fixed center, this kind of motion will be called partial.

Transition in mundane space is called real and physical motion, but if it happens within some transferred vessel, it is called relative motion.Furthermore, the transition of local motion either occurs from one place to another in mundane space i.e., the physical space of the universe, or in the relative space of some containing vessel. The former will be called real and physical motion; the latter, however, will be called relative motion, even though it often does not change its position in the place or space of the universe. Imagine, for example, a sailor walking from the prow to the stern of a ship, while the entire ship is carried by the current of a river with equal velocity in the opposite direction. It is evident that the sailor, agitated by two opposite motions equal to each other—namely, his own and that of the ship—always remains in the same position in mundane space. Then, indeed, it cannot be denied that he is truly moved, even though he is not transferred from one place of the universe to another, for he possesses transition, velocity, and impetus with respect to the ship, no differently than a motion made in mundane space has. Therefore, his migration should be called motion, but a relative one, namely, with respect to the ship itself and the air contained within it. It happens later that, upon the arrival of an opposite motion, he does not change his place or position in mundane space. It is not repugnant, therefore, that motions occur without local migration in mundane space, provided they retain a respective transition and motion. Nevertheless, it cannot be denied that in both contrary motions a true and real migration is effected in mundane space, and by this name they can be judged as true and physical motions, even though later, from the mixture of contraries, it happens that he is promoted in mundane space by this motion just as much as he is regressed and repelled by the remaining contrary motion, and thus finally remains in the same place and position.