Theme Music: Cannonball Adderly Work Song...11/23/15 Physics 131 1 Theme Music: Cannonball Adderly...
Transcript of Theme Music: Cannonball Adderly Work Song...11/23/15 Physics 131 1 Theme Music: Cannonball Adderly...
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11/23/15 1 Physics 131
■ Theme Music: Cannonball Adderly Work Song
■ Cartoon: Mike Peters Mother Goose & Grimm
November 23, 2015 Physics 131 Prof. E. F. Redish
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Mechanical energy
conservation
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Δ 12 mv
2 +U( ) = 0
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Quiz 9
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#1A 86%B 9%C 5%D 0%
#2 #3a 31% a 26%b 16% b 60%c 75% c 9%d 36% d 5%e 61% e 0%
f 1%
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Basic principle
Starting with an equation is often the right way to solve a qualitative question and remind yourself about basic principles.
11/23/15 6 Physics 131 !Fnet ⋅ Δ!r = Δ 1
2 mv2( )
A1v1 = A2v2Δp = 8µL
πR4⎛⎝⎜
⎞⎠⎟Q
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Foothold ideas: Potential Energy
■ For some forces work only depends on the change in position. Then the work done can be written U is called a potential energy.
■ For gravity, Ugravity = mgh For a spring, Uspring = ½ kx2
For electric force, Uelectric = kCQ1Q2/r12
F ⋅ Δr = −ΔU
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Using Mechanical Energy Conservation
■ If resistive forces can be ignored, mechanical energy is conserved (exchanges with hidden internal energy such as thermal or chemical can be ignored)
■ KE may refer to one or more objects PE may refer to one or more interactions.
■ If only one object’s KE is important and only one interaction matters, this can make things really easy.
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KEi + PEi = KEf + PEf
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Which Energies add to give Total Mechanical Energy?
YES Potential Energies
(Conservative Interactions)
YES Kinetic Energy
(in general, all objects)
NO Work done by Interactions
that cross System Boundary 11/23/15 Physics 131 16
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Foothold ideas: Energies between charge clusters ■ Atoms and molecules are made up of charges. ■ The potential energy between two charges is
■ The potential energy between many charges is
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U12elec = kCQ1Q2
r12
U12...Nelec =
kCQiQj
riji< j=1
N
∑