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Lect11

# Lect11 - PHYS 172 Modern Mechanics Fall 2009 Lecture 11...

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Fall 2009 PHYS 172: Modern Mechanics Lecture 11 – Energy in Macroscopic Systems Read 6.1 – 6.4

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CLICKER QUESTION #1 Reading Question (Sections 6.1 – 6.4) (This is a closed-book quiz, no consulting with neighbors, etc.) Two lead bricks moving in the +x and –x directions, each with kinetic energy K , smash into each other and come to a stop. What happened to the energy? A) The kinetic energy changed (mostly) into rest energy. B) The kinetic energy changed (mostly) into thermal energy. C) The total energy of the system decreased by an amount 2 K . D) Since the blocks were moving in opposite directions, the initial kinetic energy of the system was zero, so there was no change in energy.
Today’s Lecture • Bound States and Potential Wells • Potential energy U for Ideal Springs, Atomic Bonds • Thermal Energy

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Last Time: Planet and Star 2 2 , , , , System: planet + star 0 0 0 Rest energies constant; 0 Thus, sys surr particles system star star planet planet system star planet f system f planet i system i plan E W Q E U m c K m c K U K K U K U K ! = + = " # ! + = \$ % & " # ! + + + + = % & ! \$ + = + et system U const + =
Unbounded System: Star + Rocket E r 0 E >0 U K rocket K rocket + U = const during trip, K U • K i large enough for rocket to escape. • K i > |U i |, so that E > 0 • system is unbounded or free : separation r can be arbitrarily large U

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Bound System: Star + Rocket E r 0 E < 0 U K rocket K rocket + U = const during trip, K U rocket comes to a stop here • K i too small for rocket to escape.
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Lect11 - PHYS 172 Modern Mechanics Fall 2009 Lecture 11...

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