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nagle_phys2020_fa07_lecture8

# nagle_phys2020_fa07_lecture8 -...

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1 2 http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html Which lines are equipotentials? A) White B) Blue * Prof. Kapteyn’s office hours in the help room are 3-4 pm Friday. 3 Is this safe? 4 5 Briefly review the concepts of potential energy and work. Potential Energy = U = stored work in a system Work = energy put into or taken out of system by forces Work done by a (constant) force F θ cos | || | r F r F W v v v v Δ = Δ = Δ r F θ 6 cos | || | r F r F W v v v v Δ = Δ = Δ r F θ Δ r F θ θ < 90 degrees, Work = positive θ > 90 degrees, Work = negative 7 Lift a book by hand (F ext ) at constant velocity. F ext mg initial final h F ext = mg W ext = F ext h = mgh W grav = -mgh Define Δ U = +W ext = -W grav = mgh Note that get to define U=0, typically at the ground.

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2 8 For conservative forces Mechanical Energy is conserved. Gravity is a conservative force.
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