Because incident and reflected motion are caused between equal angles. The moving body driven by the blow gains, in every degree of time, degrees of height, and draws near to its first mover. The moving body driven by impetus gains, in every degree of its flight, degrees of distance from its mover. Therefore we shall say that whatever the moving body gains in height above the site of the percussion, that height is caused by the percussion itself; and whatever such a moving body gains in distance from the said site of percussion, that distance is caused by the impetus. Whence we shall say that a greater or lesser blow drives that moving body to a greater or lesser height, and likewise we may say that a greater or lesser impetus leaves that moving body of itself at a greater or lesser distance.

The moving body a, being moved along the line a b, is said to be moved by simple impetus. The moving body d, being moved along the line d g, will be moved by simple percussion, which is generated by the mover f descending down the line f d. Therefore in the line f d lies the power of the percussion, and in the line a b is the power of the impetus; whence it can be said with certainty that if the moving body c runs to the site of percussion d equally distant between the two said powers, it will be a motion equally sharing in the one power and the other: so that it will not rise, unless it is in a middle site, that is, sharing the height of the blow, nor has it the lowness of the impetus. There are two natures of percussion, namely when the object flees from the moving body that struck it, and when that moving body rebounds backward from the struck object. In the first, the struck object may be of equal weight and shape to the moving body that strikes it. Then in such percussion the struck object leaves in its place the moving body that struck it, and it follows the remainder of the motion that was left to the first moving body.