Lucius Gardner
Mrs. Henry
AP Physics
16 February 2016
Gravity Research Project Over the past several hundred years there have been many great scientists that have come up with significant ideas and theories in the scientific field of gravitational research. These scientists include but are not limited to Aristotle, Galileo, Kepler, Newton and Einstein! Each one of these scientist have come along after another, using each other’s ideas to further expand the human species’ knowledge on gravity and how it works. They have each left a unique mark on the scientific community. The Greek philosopher Aristotle was the first of his kind, coming up with the first theory about gravity even existing around the 4th century BC. Aristotle’s belief was that there is no effect or motion in the universe without a cause for it. He believed that heavy bodies moved downward toward the center of our universe, while light bodies moved toward the Moon. A heavy body would be something like the element of earth, not the Earth. While a light body would be something like the element of fire. Aristotle also believed that objects of different weights fall at different speeds. This gave the belief that heavier objects should fall at faster speeds than lighter objects. The next scientist to have a voice on the theory of gravity is the Italian scientist Galileo Galilei. Galileo showed that Aristotle’s assumption that the speed of falling bodies is determined by their weight is outright wrong. He said that two bodies released from a given height will reach the ground at different times not because of their different “earthly” or “fiery” characteristics but because they are affected by air friction. The argument that was used by Galileo was that a two pound rock should fall faster than a one pound rock, but if you take a two pound rock and split it in half and join these two halves with a very light sting then in one instance it should fall as fast as a two pound rock but in another instance it should fall as fast as a one pound rock. Johannes Kepler follows after Galileo in the line of contribution to our understanding of gravity. Kepler’s contributions in the early 1600s were three laws of planetary motion: The Law of Ellipses, The Law of Equal Areas, and The Law of Harmonies. The first law states that the path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus. The second law says that an imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time. And the third law of planetary motion says that the ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. Kepler’s attempts to describe his reasoning behind these motions aren’t currently accepted in the scientific community, but they do still consider these laws as accurate portrayals of the motions of planets and satellites. Sir Isaac Newton had some very important discoveries in this field of study, basing them off of the ideas of Galileo and Kepler. What Newton is probably most famous for are his three laws of motion. Newton’s first law of motion states that objects continue to move in a state of constant velocity, which can be zero, unless acted upon by an external force. Newton’s second law of motions states that the rate of change of momentum of a body is proportional to the resultant force acting on it, and will be in the same direction. Newton’s third law of motion states that the force on an object is always due to another object; all forces act in pairs that are equal in magnitude and opposite in direction. Newton also came up with the law of universal gravitation. This law stat that every mass attracts every other mass in the universe, and the gravitational force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between them. Albert Einstein is the last and possibly one of the most contributing scientists of all time, not only in the field of gravity but in many other fields as well. Most recently, Einstein’s prediction of gravitational waves from his general theory of relativity have been detected, which is a very historic discovery! Einstein’s theory is that gravity is not an ordinary force, but that it is a property of space-time geometry. This was a very alternate theory to Newton’s Law of Universal Gravitation. In the theory that Newton came up with, two objects that begin in a parallel path collide because of gravity and gravity is therefore described as a force. In Einstein’s theory, objects still travel in the straightest line possible and the collision is still caused by gravity but gravity is a distortion in space-time. Modern day scientists are working everyday just as these scientists did to prove or disprove of theories from past scientists in the field of gravity. As a matter of fact, a very historic discovery based on one of Einstein’s theories was proved last week! Even though scientists are coming up with new theories for gravity, we still acknowledge and accept past ones discovered by those who have gotten us where we are on gravity today. The technology of our time has given modern scientists a very big advantage and has made it easier for us to prove or disprove these theories.

Bibliography 1. Wudka, Jose. "The Motion of Falling Bodies." Physics.ucr.edu. N.p., n.d. Web. 15 Feb. 2016. 2. O'Connor, J. J., and E. F. Robertson. "History Topic: Theories of Gravitation." Gravitation. N.p., 2003. Web. 15 Feb. 2016. 3. "Kepler's Three Laws." Kepler's Three Laws. The Physics Classroom, n.d. Web. 15 Feb. 2016. 4. "Newton's Theory of Gravity." The Star Garden. N.p., n.d. Web. 15 Feb. 2016. 5. Redd, Nola Taylor. "Einstein's Theory of General Relativity." Space.com. N.p., n.d. Web. 15 Feb. 2016.