Aristotelian Physics. Aristotle's physics agrees with most people's common sense, but modern scientists discard it. So what went wrong?

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1 Aristotelian Physics Aristotle's physics agrees with most people's common sense, but modern scientists discard it. So what went wrong? Here's what Aristotle said:

2 Aristotelian Physics Aristotle s classification of motion Natural motion every object in the universe has a proper place determined by a combination of four elements: earth, water, air, and fire any object not in its proper place will strive to get there examples: stones fall puffs of smoke rise

3 Aristotelian Physics Natural motion (continued) straight up or straight down for all things on earth beyond Earth, motion is circular example: Sun and Moon continually circle Earth Violent motion produced by external pushes or pulls on objects example: wind imposes motion on ships

4 Aristotelian Physics Aristotle said that a heavier object should naturally fall much faster than a lighter object. With modern technology, it is easy to demonstrate that a feather and a rock fall at exactly the same rate in a vacuum tube. There was a man who lived 400 years ago who didn't need our modern technology to figure out that Aristotle was wrong. Who was he?

5 Galileo's Law of Falling: Aristotle had said that heavy objects should fall noticeably faster than lighter objects. Galileo was the first scientist to challenge him on this! If air resistance is negligible, then any two objects that are dropped together will fall together, regardless of their weights and their shapes, and regardless of the substances of which they are made.

6 Galileo correctly identified the influence of the force of friction on objects. When objects move they usually rub against a surface. They can even rub against the air. This rubbing force, which opposes their motion is called friction. To truly observe an object in its natural state of motion, as Aristotle envisioned, you would have to get rid of the interfering influence of friction.

7 In principle, it is next to impossible to get rid of all friction, but, as technology has advanced, we can now get pretty close. When you drop a feather and a stone in the air, the stone obviously falls faster. Galileo would say that friction with the air is holding back the feather. What would Aristotle say? When you drop a feather and a stone in a vacuum, they fall together at the same rate. Unfortunately, Galileo could not do this kind of demonstration in a vacuum.

8 However, it is alleged that Galileo did do another kind of demonstration where the effects of friction with the air could be minimized. It is said that Galileo dropped a 10 kilogram cannonball and a 1 kilogram stone from the leaning tower of Pisa. Despite the fact that the cannonball was 10 times heavier, it hit the ground at nearly the same time as the stone. The acceleration of gravity should be the same for both.

9 Galileo observed that a ball rolling down a steep ramp picks up speed quickly. A ball rolling down a ramp that is less steep still picks up speed but at a slower rate.

10 Galileo reasoned that if you were to start a ball moving on a horizontal surface, ideally it should never speed up or slow down. It would keep on moving forever. Of course, there would have to be no friction. Today, we can come close to this by observing objects in outer space or on low friction surfaces such as ice.

11 In the absence of friction, a ball rolling down the incline on the left tends to roll up to its initial height on the right.

12 Galileo also sought to know what an object's natural state (or tendency) of motion should be. Unlike Aristotle, Galileo came to a different conclusion. Galileo said that an object in motion will tend to keep on moving in a straight line at constant speed unless there was some unbalanced force (like friction) acting upon it. This is the Law of Inertia

13 Galileo was one of the first to practice what we now call the scientific process. His methods include: 1. Experiments designed to test ideas. 2. Idealizations to eliminate side effects that could hide the truth. 3. Limiting the scope of the inquiry. (One question at a time.) 4. Quantitative methods. He was not afraid to use math.