# Physics 1: 3.20

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• There once lived a man in an ancient society. He had some views that others didn't agree with about astronomy.
• Others believed theories about the universe that differed from his own beliefs.One of these theories would be the "geocentric" universe. This involved the Earth being the center of the universe with other planets orbiting around it. Each orbit was in uniform circular motion. This means that each body would travel a constant speed in a circular path.
• Aristotle
• An example of uniform circular motion would be a wheel on a car turning at a continuous 60 mph.
• Fcentripetal = m(v^2/r) This is the equation for centripetal force. Centripetal force = mass x centripetal acceleration
• The man stayed up countless nights trying to figure out the universe and all of its secrets.
• What makes planets orbit? What is the centripetal force? This force causes planets to follow a circular orbit around SOMETHING. What is it?
• The man thought that if he had some way of measuring the tangential velocity of the planets' orbits then he could prove other theories wrong! The planets don't travel in uniform circular motion! And they don't travel around the Earth!
• Vt = ω r (This is the formula for tangential velocity.) tangential velocity = angular velocity x the radius of the "wheel"
• Eventually other scientists of the day convinced the public of the heliocentric model of the universe. The man still had many questions about the universe and how it worked. One of those questions was: "how can I find how round the Earth is and compare it to the Sun?" This would involve finding the circumference of both bodies.
• 2πr This is the formula for circumference. 2 x pi (3.14) x the radius of the object
• While the man didn't have all of the tools to answer some of his questions, with modern technology and the work of scientists, the answers are now just a click away!
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