According to Aristotle, when one drops a ball, the ball falls straight down to the ground, because its “natural motion” seeks the center of the Earth. The ball’s natural motion is straight down – not curved, not sideways, not diagonal, but always directly toward the center of the Earth. This natural motion remains the true path of the ball even if the person dropping it is in motion. The person’s motion, according to Aristotle, should have no continuing effect upon the ball after they are no longer in contact. Suppose that one is running with the ball at the moment the ball is let go. Once released from the runner’s hand, the ball should fall straight down with its own natural motion; whether the runner was in motion or not should make no difference to the ball. This is Aristotle’s account of the natural motion of falling bodies.
Now if Aristotle’s reasoning were true, one would predict that a cannon ball dropped by a sailor from the crow’s mast of a fast-moving ship would splash into the water far behind the ship, for the ship would move out from underneath the ball before enough time had elapsed for the ball to fall to the surface of the water. Aristotle carried out experiments, so it only stands to reason (one might argue) that many Aristotelians carried out this experiment. So why do we have no eye-witness reports refuting this Aristotelian prediction?
My friend Michael Barfield, who frequently posts clever cartoons on This Visual Feast, posted the answer today:
Michael Barfield, inertia cartoon from This Visual Feast
Wonderfully funny! Click the image to go to the original at This Visual Feast and look carefully at the undersea landscape in the last panel.
PS: In the 14th century, Nicole Oresme posed a thought experiment involving ships to demonstrate the relativity of motion and the nature of “impetus,” contrary to Aristotle. Here’s Oresme’s statement of Aristotle’s position, which he rejects in the very last sentence (bolded):
“If a person on a ship moved rapidly eastward and an arrow were shot directly upward, it ought not to fall on the ship but a good distance westward from the ship. Similarly, if the Earth is moved so swiftly in turning from west to east, and it has been posited that one throws a stone directly above, then it ought to fall, not on the place it left, but a good distance to the west. But in fact the contrary is clear.”
(Nicole Oresme, 14th century; read more…)
17th-century investigations of inertia by Galileo, Descartes and Newton represent the culmination of a long line of investigations reaching back through medieval Islamic and European civilizations to Basil of Caesarea and John Philoponos in late antiquity.
Undoubtedly, like Basil, Philoponos, and Oresme, sailors throughout history knew good and well that Aristotle’s account of falling bodies was wrong. This explains why the Captain depicted in the first panel of the cartoon has gone mad.
To hear a report of an experiment normally requires the witnesses to survive to tell about it.