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chapter26

# chapter26 - Chapter 26 Relativity Basic Problems The speed...

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Chapter 26 Relativity

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Basic Problems The speed of every particle in the universe always remains less than the speed of light Newtonian Mechanics is a limited theory It places no upper limit on speed It is contrary to modern experimental results Newtonian Mechanics becomes a specialized case of Einstein’s Theory of Special Relativity Applies when speeds are much less than the speed of light
Foundation of Special Relativity Relating of the measurements of two observers moving relative to each other Normally observers measure different speeds for an object Special relativity relates two such measurements

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Galilean Relativity Choose a frame of reference Necessary to describe a physical event According to Galilean Relativity, the laws of mechanics are the same in all inertial frames of reference An inertial frame of reference is one in which Newton’s Laws are valid Objects subjected to no forces will move in straight lines
Galilean Relativity Example A passenger in an airplane throws a ball straight up It appears to move in a vertical path This is the same motion as when the ball is thrown while at rest on the Earth The law of gravity and equations of motion under uniform acceleration are obeyed

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Galilean Relativity Example, cont There is a stationary observer on the ground Views the path of the ball thrown to be a parabola The ball has a velocity to the right equal to the velocity of the plane
Galilean Relativity Example, conclusion The two observers disagree on the shape of the ball’s path Both agree that the motion obeys the law of gravity and Newton’s laws of motion Both agree on how long the ball was in the air Conclusion: There is no preferred frame of reference for describing the laws of mechanics

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Galilean Relativity Limitations Galilean Relativity does not apply to experiments in electricity, magnetism, optics, and other areas Results do not agree with experiments The observer should measure the speed of the pulse as v+c Actually measures the speed as c
Luminiferous Ether 19 th Century physicists compared electromagnetic waves to mechanical waves Mechanical waves need a medium to support the disturbance The luminiferous ether was proposed as the medium required (and present) for light waves to propagate Present everywhere, even in empty space Massless, but rigid medium Could have no effect on the motion of planets or other objects

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Verifying the Luminiferous Ether Associated with an ether was an absolute frame where the laws of e & m take on their simplest form Since the earth moves through the ether, there should be an “ether wind” blowing If v is the speed of the ether relative to the earth, the speed of light should have minimum (b) or maximum (a) value depending on its orientation to the “wind”
Michelson-Morley Experiment First performed in 1881 by Michelson

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