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Lecture_31 - PHYS 342 Fall 2011 Lecture 31 Introduction to...

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PHYS 342 Fall 2011 Lecture 31: Introduction to Relativity Ron Reifenberger Birck Nanotechnology Center Purdue University L t 31 1 Lecture 31
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Relativity This word is usually associated with Einstein, but……………really it is related to a 2,500 year long effort to understand our place in the universe: ~ 500 BC - Earth was at rest at the center of a flat universe that was constructed with a mathematical plan ~ 1500 – suggestion that Earth actually moves around the sun the universe ~ 1900 - no center to the universe, there is no place in the universe that is at rest, the universe is not flat. Q: What is Relativity? A: Relativity is the idea that the laws of the universe are the same no matter what direction you are facing, no matter where you are standing, no matter how fast you are moving, so long as the speed you are moving is constant. 2
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In Galileo’s time (early 1600’s) this was THE problem of the century Scientific origin of Relativity “Does the earth move?” In Galileo s time (early 1600 s) this was THE problem of the century. Arguments against earth motion: There is NO speed sensation when standing on the surface of the earth. If the earth were moving (caused by say the Earth’s rotation), anything that one dropped would rapidly fall behind and drift backward anything that one dropped would rapidly fall behind and drift backward. Galileo’s counter argument – a simple thought experiment: Put a man below deck on a ship. Is the ship docked or is it moving smoothly through the water? Look around!! - - - - water drips, fish swim in a tank, butterflies fly about, pendulums swing back and forth and so on; these phenomena all look the same whether swing back and forth, and so on; these phenomena all look the same whether the ship is moving at a constant speed or is standing still. Galileo concludes you can’t tell if the earth is moving so long as it moves smoothly! ( h l d h d f f l f f f d h 3 (This leads to the definition of an Inertial frame of reference and the notion that the Laws of Physics are the same in all inertial reference frames)
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Frame of Reference A convenient way to identify an inertial frame of reference is when ALL the laws of physics take their simplest form . Pick a reference frame, then apply Newtonian mechanics to explain the lt f i t Wh t f d N t ' L f M ti t k t results of an experiment. What form do Newton's Laws of Motion take to explain the experimental observations?
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