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Physics 40 |
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Chapter 5 -- Laws of Motion |
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In a two ball juggling, one ball leaves the hand
as the other reaches hmax, the top of the arc |
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At what fraction of hmax do the balls
have the same height? |
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(a)
1/2 (b) 1/3
(c) 2/3 (d)
3/4 |
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3/4 hmax |
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Remember Galileo’s law of odd numbers |
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Force and acceleration will be related by the
idea of mass |
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Universal gravitation |
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Free fall and air friction |
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Chalk into the pop bottle |
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Laws of motion |
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Galileo said a = 0 iff F = 0 |
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Newton took it a bit further |
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Law of gravity |
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Suggested inverse square law |
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Orbits |
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Worked out motion of moon and planets |
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Adds a bit to F = 0 iff a = 0 |
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Seems a small bit more to say F and a are
proportional |
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F = ma |
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One of the most remarkable models of the real
world ever proposed |
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Special value of acceleration in 2nd
law |
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F = ma |
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is g, the acceleration due to gravity. |
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If a = g, then the force is called “weight” |
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W=mg |
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Force is only half an interaction |
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For every force of action Fa, there
is an equal and opposite force of reaction Fr. |
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Or |
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Fa = - Fr |
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Really, a statement about the nature of this
abstraction we call “force.” |
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All objects attract all other objects |
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Inversely proportional to square of distance |
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Directly proportional to masses |
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Depends on mass and radius of planet. |
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Planet X problems |
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Big Planets |
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Little Planets |
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From g and W we can calculate Mearth from Rearth |
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so |
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“Free fall” has different meanings |
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Skydiving |
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Notes