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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| De motu antiquiora | 4/12 | https://en.wikipedia.org/wiki/De_motu_antiquiora | reference | science, encyclopedia | 2026-05-05T08:51:23.985272+00:00 | kb-cron |
=== Chapter 7: The cause of speed and slowness of natural motion === Differences in speed of motion occur in two ways: either the same body moves in different media, or different bodies move through the same media; furthermore, the case in which different bodies move in different media can be simplified down to these two ways. In both cases, the speed of motion depend upon the same cause: the greater or lesser weight of the media and the moving bodies. In this argument, Galileo focuses on the primary cause of motion rather than any “accidental” (e.g., the shape of the body) or external cause. Aristotle claimed that the same body moves more swiftly in a rarer medium than in a denser medium, and that therefore the cause of slowness of motion is the density of the medium, and the cause of speed is rareness of the medium; however, Galileo points out that Aristotle asserted this claim on the basis of no other reason than from experience (i.e., we see a moving body move more swiftly in air than in water) and that this explanation of the cause to produce this effect is insufficient since there are many moving bodies that move more swiftly with natural motion in denser media than in rarer ones. For example, an inflated bladder of air falls slowly in air but rises quickly in water. Furthermore, Galileo notes that, for downward motion, a heavier substance moves more swiftly than a lighter, and for upward motion, the lighter substance will move more swiftly. In the case of motion of the same body moving in different media, a body that is heavier than a medium will move more swiftly in the medium than in another medium in which the body is less heavy; moreover, a body that is lighter than a medium will move more swiftly in the medium than in another medium in which the body is less light. Therefore, if we find in what media a given body is heavier, we shall have found the media in which it will fall more swiftly. Furthermore, if we can find how much heavier the same body is in one media than another, we will know how much faster it will move.