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Aristotle's Theory
Aristotle was one of the first people to realize that a force existed in the world which made things fall. He believe that all objects fell toward their natural place; earth, but that heavier things fell at a faster aceleration. (http://en.wikipedia.org/wiki/Aristotelian_physics) -
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Ptolemy's Theory
His theory was based around the the same idea that aristotle's was based on; that the earth was in the center of the universe and that everything fell toward it. To solidify his theory he created a complicated expeiment using circles and epicycles which helped him to explain all the movement in the sky. When something did not work out, he would add another epicycles. (http://www-scf.usc.edu/~kallos/gravity.htm) -
Newton's Theory of Universal Gravitation
When Newton saw an apple fall from a tree, he wondered if the same force existed on the moon. He believed that since the moon never moved farther from the earth, and had an orbit that cloesly resembled a circle, the earth must exert a pull on the moon to keep it from going astray. He named the force "gravity." He also believed that all the planets circled the sun because it held them all in place. He also did the cannon experiment. ( http://www-istp.gsfc.nasa.gov/stargaze/Sgravity.htm) -
Cavendish's Theory
In one of his most famous experiments, known as the Cavendish experiment, he was able to measuer the density of the earth using a device built by english geologist John Michell. He measuered the gravitational attraction between two lead balls and found that the density of earth was 5.48 times as greater than the density of water. This helped Newton in the Gravitational equation. (http://en.wikipedia.org/wiki/Henry_Cavendish) -
Einstein's theory of relativity
In Einstein's theory, gravity was a property of space and time or spcetime. While it is not the only theory on this subject, it is the simplest theory that corresponds with the data provided by Newtons experiments. (http://en.wikipedia.org/wiki/General_relativity)