405 the cause was sought why natural motion is faster at the end than at the middle, and faster in the middle than at the beginning. Thus we must recall what was made clear above; namely that a mobile, when it is moved with a violent motion, is moved up to the point where the force impressed by the motor is greater than the resisting heaviness: from this it follows that when a heavy thing ceases to go up, the force impressed in it is equal to its heaviness; from this it clearly follows that, when a heavy thing begins to be moved downward, it is not moved with a motion that is purely and simply natural. For at the beginning of such a motion there is still in this mobile some of the impressed force which was impelling it upward: this force, because it is smaller than the heaviness of the mobile, does not impel it any farther upward; however it still resists the heavy thing that is heading downward, because it has not yet been annihilated.For it has been demonstrated that it is successively weakened: and thus it happens that the mobile at the beginning of its own natural motion is moved slowly: but afterwards, since the contrary force is weakened and diminished, the moving thing, encountering lesser resistance, is moved more swiftly. As if, for example, we imagine a mobile which is moved with a violent motion from a to b, whose heaviness is 4, it is manifest that the force which impels it will be greater than 4 at any point of the line ab described by the forced motion: but at b itself it will not be greater than 4 (for if it were greater, the same mobile would be impelled by it beyond b); also it will not be smaller (for it would have been equal to it before [the body was at] b; but it has been demonstrated that it has always been greater); hence the force at b will be equal to the heaviness of the mobile, namely it will be 4.Therefore, when the mobile recedes from b, the force which was as 4 begins to weaken, and, on account of this, the mobile begins to have a lesser resistance to its own heaviness; because this resistance is continuously weakened, it results that the natural motion is continuously intensified.
DO. This solution more than pleases me: however it seems to have its place only in the case of a natural motion preceded by a violent one. But when someone having a stone in hand does not impel it upward, but only lets go of it, in this motion, which is not preceded by a violent [motion], what will be the cause of the intensification? {1}
AL. This very doubt had also come to my mind when I was working out an explanation to this problem; but when I examined the matter more carefully, I discovered that it was of little importance. Thus the intensification occurs by the same cause in each of the two motions [downward], as much in that which is preceded by a state of rest, than in that [of a violent motion {1}]