12_gravity - Lesson 12: Gravity Aristotle From the time of...

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Lesson 12: Gravity Aristotle From the time of Aristotle (384-322 BC) until the late 1500’s, gravity was believed to act differently on different objects. This was based on Aristotle's observations of doing things like dropping a metal bar and a feather at the same time. Which one hits the ground first? Obviously, common sense will tell you that the bar will hit first, while the feather slowly flutters to the ground. In Aristotle’s opinion, this was because the bar was being pulled harder (and faster) by gravity because of its physical property of having more mass. Because everyone could see this when they dropped different objects, it wasn’t questioned for almost 2000 years. Galileo Galileo Galilei was the first major scientist to refute (prove wrong) Aristotle’s theories. In his famous (at least to Physicists!) experiment, Galileo supposedly went to the top of the leaning tower of Pisa and dropped a wooden ball and a lead ball at the same time. Both were the same size, but different masses. Down below an assistant watched for them to hit the ground. They both hit the ground at the same time , even though Aristotle would say that the heavier metal ball should hit first. Galileo had shown that the different rates at which some objects fall is due to air resistance, a type of friction. Get rid of friction (air resistance) and all objects will fall at the same rate. Galileo said that the acceleration of any object (in the absence of air resistance) is the same. To this day we follow the model that Galileo created. a g = g = 9.81m/s 2 a g = g = acceleration due to gravity Since gravity is just an acceleration like any other, it can be used in any of the formulas that we have used so far. Just be careful about using the correct sign (positive or negative) for the variables in the problem. I would strongly suggest you always stick with up being positive, and down being negative. 7/22/2010
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This note was uploaded on 12/02/2011 for the course PHYSICS 235 taught by Professor Staff during the Fall '08 term at Rutgers.

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12_gravity - Lesson 12: Gravity Aristotle From the time of...

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