Lecture5 - Physics 7A Lecture 5 Winter 2010 Prof. Robin D....

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Unformatted text preview: Physics 7A Lecture 5 Winter 2010 Prof. Robin D. Erbacher 343 Phy/Geo Bldg erbacher@physics.ucdavis.edu Next Monday is a HOLIDAY. No lecture. DLs canceled M-W. Check calendar for specifics. Quiz 2 rubric is posted, all grades will be shortly. Quiz 3 is today, focusing on material from DLMs 5-9. Join this Class Session with your PRS clicker! Check Physics 7 website frequently for updates. Turn off cell phones during lecture. Three New Energy Systems From the conservation of energy: In the case of the mass-spring, we have both PE and KE: PE m-s + KE translational = Q+W k ( y 2 ) + m ( v 2 ) = Q+W E total = DE 1 + DE 2 + DE 3 + ... = Q + W Displacement from equilibrium y [+] [-] PE mass-spring Displacement from equilibrium y [+] [-] PE mass-spring Clicker: If the mass is displaced upwards, the following is true: a)The dot moves up and to the right, and the force vector points to the left. b)The dot moves up and to the right, and the force vector points to the right. c)The dot moves up and to the left, and the force vector points to the right. d)None of the above. Displacement from equilibrium y [+] [-] direction of force y PE mass-spring Displacement from equilibrium y [+] [-] direction of force PE mass-spring Displacement from equilibrium y [+] [-] PE mass-spring On this side force pushes up On this side force pushes down Equilibrium Forces from potentials point in direction that (locally) lowers PE Displacement from equilibrium y [+] [-] PE mass-spring Equilibrium Potential Energy curve of a spring: Putting work into the system increases the energy. Here, work is force through a distance Displacement from equilibrium y [+] [-] PE mass-spring Equilibrium Notes Force is always in direction that locally decreases PE Force is related to the slope -- NOT the value of PE Attractive forces cause the particles to come together (PE decreasing as r decreases) Repulsive forces cause the particles to separate (PE decreases as r increases) The steeper the PE vs r graph, the larger the force ~Force Why does it take more...
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Lecture5 - Physics 7A Lecture 5 Winter 2010 Prof. Robin D....

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