have understood the things said above, we deliberately omit them: only noting the following, that, just as has been said above concerning vertical motion, so also in the case of these motions on planes it happens that the ratios that we have put forward are not observed, sometimes for the reasons of the causes just now alleged, sometimes -- and this is accidental -- because in the beginning of its motion a lighter mobile goes down more swiftly than a heavier one: why this happens, we will make clear in its proper place {2}; for this question depends on the one in which it is asked why the swiftness of natural motion is augmented {3}.</s>
<s id="id.1.2.1.10.04">But, as we have often said, these demonstrations presuppose that there are no extrinsic impediments, either from the shape of the mobile, or else from the roughness of the plane or the mobile, or from the motion of the medium in the opposite or in the same direction [as that of the body], or else from an extrinsic moving force quickening or retarding the motion, and other similar things: for concerning these accidents, since they can happen in countless ways, rules cannot be given. </s>
<s id="id.1.2.1.10.05">One must consider in a similar fashion the case of upward motion. </s>
</p>
<p>
<s id="id.1.2.1.11.00"/>
<s id="id.1.2.1.11.01">Let these things suffice concerning motion on inclined planes. </s>
<s id="id.1.2.1.11.02">Now it remains that we say something in the next chapter concerning circular motion: asking, first, whether or not it has a ratio to rectilinear motion, and whether it is forced or natural motion. {1} </s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.1.2.2.00.00"/>
<s id="id.1.2.2.00.01">Older Works on Motion, Book II, chapter 2 [15] </s>
