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Unformatted text preview: Lecture 17 Energy Diagrams ACT: Force field The pictures below show force vectors at different points in space for two different forces. Which of these forces is conservative? A. 1 B. 2 C. Both 1 2 W = 0 W 1 W = 0 W 3 W 2 W = 0 1 2  W 2 + W 3 =  W 1  W loop = 0  W 2  >  W 1  W loop Work along a closed trajectories (loops). W = 0 W 2 W = 0 W 1 Relation between Relation between U and and F (1D) (1D) final initial x W F d x U = = For a conservative force (in 1D), constant x U F d x = + x dU F dx = 2 Examples: 1 2 y x dU U mgy F mg dy dU U kx F kx dx = = = = = = The force is minus the slope of the U ( x ) curve. Example: Spring 2 1 ( ) 2 U x kx = x U x x = 0, F = 0 dU/dx = 0 Potential energy of a box attached to a spring on a horizontal, frictionless table. x U x x < 0, F > 0 dU/dx < 0 x U x x > 0, F < 0 dU/dx > 0 x U x x > 0, F < 0 and larger in magnitude dU/dx > 0 and steeper than before The force always points downhill!!! ACT: ACT: U vs vs F A Which of the force versus position graphs matches the potential energy function shown in blue? U x F x x x F F B C + Slope: Force = slope Relation between Relation between U and and F (3D) (3D) x U F x = 1D: , , in cartesian coordinates for the radial component in spherical coordinates x y z r U U U F F F x y z U F r = = = = 3D: (minus the gradient of...
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This note was uploaded on 04/16/2008 for the course PHYSICS 221 taught by Professor Johnson during the Fall '06 term at Iowa State.
 Fall '06
 Johnson
 Physics, Energy, Force, Work

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