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Lect13-1

# Lect13-1 - PHYS 172 Modern Mechanics Fall 2009 Lecture 13...

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Fall 2009 PHYS 172: Modern Mechanics Lecture 13 – Friction, Driven Oscillations, and Resonance Read 6.10 – 6.14

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CLICKER QUESTION #1 Reading Question (Sections 6.10 – 6.14) (This is a closed-book quiz, no consulting with neighbors, etc.) You throw a ball straight up, and when it comes down you catch it. Consider the round trip from when it left your hands to when it returned to the same height (before being caught). Choose the correct answer: positive negative D positive zero C negative zero B zero positive A Work done by air Work done by gravity
NEXT WEEK: No labs No lectures There is still recitation Exam #2 is Wednesday, October 14 in Elliot Hall at 8 pm Same format as before (all about Energy!) Bring pencils, ID, calculator (graphing is ok) Alternative testing in room 8 of Physics building at same time

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Today’s Lecture • Friction and Energy Dissipation • Driven Oscillations and Resonance
A horse of mass M gallops with constant speed v up a long hill of height h and horizontal extent d . Choose the horse as the system to analyze. Start from the energy principle: ! E sys = W ext + Q We found that: 1. Forces are exerted on the system by the Earth (gravity), the ground (normal force), and air (resistance, friction). 2. The normal force does no work, because there is no displacement of the hooves. 3. The work done by the Earth is -Mgh. 4. Q<0 because the horse is hotter than the air, so heat energy leaves the system. ! E sys = W ext + Q = " Mgh " Q Recap of galloping horse:

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Concerning the energy of the system (horse): 1. E= Mc 2 + K + E chemical + E thermal 2. Only E chemical will change chemical E Mgh Q ! = " " sys E W Q ! = + We see that the stored chemical energy of the horse must decrease.
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