The Aristotelian model of nature seemed to account for normal observations. In addition, it harmonized with philosophical ideas about the distinction between the heavens and the Earth. These ideas were so strong that they were accepted for 2000 years without any basic questions. However, with the Copernican model the seeds of questions were sown. With the new findings of Kepler and Galileo, not only was the Aristotelian view of the heavens being questioned, but all of the mechanics espoused by Aristotle were being opened to testing.
After being sentenced to house arrest, Galileo did not stop working on science. The last great book written by him was Discourses and Mathematical Demonstrations Concerning Two New Sciences Pertaining to Mechanics and Local Motion. Aristotle had argued that if a heavy object and a light object are dropped at the same time, the heavier one will reach the ground before the light one. Galileo stated that experimentation with two stones, one weighing ten times the other, showed that they fall side by side, at least as far as can be observed.
One might argue that the heavier object is only 10 to 20 times heavier than the lighter one and that if they were more different in weight they would surely not fall together; that a grain of sand would not fall as rapidly as a concrete block. Galileo's reply to arguments like this was that although you might be able to see a small difference in the rate of fall of two objects that are very different in weight, Aristotle had argued that if one object is 1000 times heavier than another, it should fall 1000 times as fast. This is obviously not true. Aristotle's idea, then, must be wrong.
But what about Galileo's statement that objects fall side by side? Galileo admitted that we might be able to tell some difference in the rate of fall of objects that differ greatly in weight, and he had an answer for why this happens. It happens for the same reason that a feather falls slower than a rock- air resistance is involved. (Check out this fictitious newpaper account of the "fabled" experiment on motion supposedly conducted on the Tower of Pisa.)
Galileo proposed that it is the air that prohibits a piece of paper from falling faster; that the air has much more effect on the feather than it does on the rock. Likewise, any difference between the falling times of the grain of sand and the concrete block are due to the effects of air friction. If we could drop these objects in a perfect vacuum where there is no air to affect them, they fall exactly together. This experiment was performed on the Moon during the Apollo program. Here, a hammer and a feather were dropped side by side, Since there is no air on the Moon, then according to Galileo, they should land at the same time. And indeed, when the experiment was conducted, the astronaut was heard to say "What do you know, Galileo was right!"
Before going on, we need to take a quick side trip into physics. More specifically, we need to borrow the definitions of speed, velocity and acceleration. Recall that, until the time of Galileo and Kepler, descriptions of nature relied primarily on geometry. However, Kepler described the relationship between a planet's period and its average distance from the Sun using an equation. Similarly, Galileo used equations in his descriptions of nature. Because of this, he needed to define his concepts carefully, in a manner that permitted the quantities to be measured and related mathematically to one another. In the physical sciences today, such definitions are a necessity.
From the lecture
notes of Dr. Daniel Suson, associate professor of physics at Texas A & M
University, Kingsville. Notes found at http://newton.tamuk.edu/~suson/html/1411/physics.html#Galilean
Click here to read a fictitional newspaper account of Galileo's fabled experiment at the
Tower of Pisa. Myth has it that Galileo conducted
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