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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| De motu antiquiora | 10/12 | https://en.wikipedia.org/wiki/De_motu_antiquiora | reference | science, encyclopedia | 2026-05-05T08:51:23.985272+00:00 | kb-cron |
=== Chapter 17: The agency by which projectiles are moved === Aristotle argues that objects move due to contact with a mover, but since projectiles fly without contact from a mover, then it must be that the rushing of air behind the object is propagating its motion. Galileo raises several objections to this explanation (most of which were recognized much before Galileo): the successive parts of air that push the projectile would always be accelerated, which is contrary to Aristotle’s assumptions; experience shows that arrows fly despite a strong opposing headwind; a ship propelled by oars against a current continues to move forward long after the oars are retracted from the water; iron balls can be flung at a great distance, and yet flaxen fibers fall to the ground sooner than the iron ball; lastly, a marble sphere can spin for a long time without displacing, thus leaving no space for air to push against it, nor is a flame placed underneath the sphere disturbed by any air currents. Instead, Galileo argues that projectile motion results from an impressed force that gives the projectile a self-depleting impetus for its motion (as a side note, according to Drabkin, medieval philosophy historian E. A. Moody "sharply differentiates the development of Galileo’s theory of impressed force from Jean Buridan’s impetus theory"). Thus, this impressed force or impetus modifies the heaviness of a body when thrown upward, and the lightness of the body when thrown downward. Galileo analogizes this impressed force to a temperature of a body, such that when a mover acts upon the body, it is much like placing iron in a fire, and once the projectile has left the hand of the mover, the impressed force diminishes much like how iron, once pulled from the fire, loses its heat and returns to its natural coldness. As to where in the projectile the impressed force is received, Galileo shifts the onus of the question by stating that the impressed force is wherever one believes heat is stored in the projectile. He then compares the impressed force transferred from a mover to the mobile much like what is transferred from a hammer to a bell: initially both silent, the hammer impacts and imparts a sonorous quality to the bell which is contrary to its natural silence, and over time the sound gradually diminishes, much like an impressed force applied to a projectile. Moreover, Galileo argues that it’s not the air that continues to strike the bell to produce the sound, but rather the bell continuing to vibrate due to the impressed force received from the hammer. He then elaborates further regarding the modification of weight due to the impressed force: although the body becomes lighter from the impressed force, the effect is only temporary, and throughout its motion the body still retains its natural weight while the modified heaviness or lightness diminishes over time. Furthermore, heavy objects tend to retain their impetus for a longer time than lighter objects. Galileo then discusses how certain opinions, however false they may be, remain persistent because, at first sight, they offer some appearance of truth, but no one bothers to examine whether they are worthy of belief. He offers an example of a common misconception in which it is believed that water itself causes objects in it to appear larger. He says that he experimented with the phenomenon and found that a coin deep in water did not appear larger at all, but perhaps instead smaller. Galileo concludes that the misconception possibly arose due to the decorative tradition of placing fruits in a glass vessel filled with water in the summer, and that it was the curvature of the vessel that caused this largening effect and not directly because of the water. He then concludes the chapter by emphasizing that it is certain that projectiles are in no way moved by the medium, but only by a motive force impressed by a mover.
=== Chapter 18: A demonstration that the motive force is gradually diminished in a moving body === Galileo argues that forced motion is not endless, that the force is continuously diminished in the projectile, and that motion with persistent speed and constant motive force “surely is most absurd.” Galileo would later abandon this erroneous idea through his implicit discussions regarding inertia in his Two New Sciences.