Lecture04_10 - Physics19 GreatIdeasofPhysics Lecture4:

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Click to edit Master subtitle style Physics 19 Great Ideas of Physics Lecture 4: From Xeno to Galileo
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Why Study Mechanics? Describing how  things move (kinetics) is  necessary to  understand why  they move as  they do (dynamics) Mechanics forms the basis for energy and  heat Before Galileo and Newton, these seemingly  trivial ideas were not understood After Einstein and Quantum Mechanics, it 
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Click to edit Master subtitle style Xeno and Aristotle
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Xeno’s Paradoxes Mechanics appears trivial, but it took 2000  years to understand! One set of Greek philosophers, called  “Sophists” confused almost everyone! Xeno’s paradoxes: The Dichotomy Achilles and the Tortoise
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Xeno’s Paradox: The Dichotomy Xeno was a “sophist”, a word which (today) means  “someone who talks confusing nonsense”! Xeno and the Sophists argued that change was impossible For instance, it should take forever to walk across the room First, you have to go halfway But before you can go halfway, you have to go ¼ of the way But before you can go ¼ of the way, you have to go 1/8 of the way,  etc It takes an infinite number of steps to travel any distance  whatsoever! So (according to Xeno) it is logically impossible for you to move at 
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Achilles and the Tortoise Another of Xeno’s paradoxes involved a race  between Achilles (supposedly the fastest human)  and a tortoise Because he is so much faster, Achilles gives the  tortoise a head start According to Xeno, Achilles can never pass the  A T T A A T
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Aristotle and Xeno’s Paradoxes Xeno had a four paradoxes related to motion They were all explained and shown to be  nonsense by Aristotle in his book “Physics” But Aristotle didn’t quite understand motion  either…
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Aristotelian Physics In Aristotle’s Physics, the speed of an object  is determined by: The “weight” of the object (how much there is of  it determines how quickly it moves to its “natural”  place) The “resistance” of the medium it travels in Thus, according to Aristotle: Speed v — W / R
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According to Aristotle, a heavier object does  not “accelerate” – it acquires all its speed   instantly Clearly, Aristotle was not much of an  experimentalist! According to Aristotle, a heavier object will 
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Lecture04_10 - Physics19 GreatIdeasofPhysics Lecture4:

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