Conservative and Non-Conservative Forces Hey—where did the energy go? § 6.6

Poll Question The piglet has a choice of three frictional slides to descend. Along which slide would gravity do the most work on the piglet? A B C The wgrav is the same for all. 2

Poll Question The piglet has a choice of three frictional slides to descend. Along which slide would friction do the most work on the piglet? A B C The wf is the same for all. 3

Poll Question The piglet has a choice of three frictional slides to descend. Along which slide would the piglet have the greatest final speed? A B C The vf is the same for all. 4

Classes of Forces Conservative forces Non-conservative forces maintain mechanical energy (U + K) Non-conservative forces change mechanical energy

Conservative Force Path independence Position dependence The work done by a conservative force on a body moving from Point A to Point B is: The same along every path from A to B. Zero along every closed path (loop). Position dependence A conservative force is the rate of change of potential energy with position.

Question The force of gravity F = mg is: Conservative. Non-conservative.

Question The force of a Hooke’s law spring F = –kx is: Conservative. Non-conservative.

Question The force of kinetic friction F = mkN is: Conservative. Non-conservative.

Question The force exerted by a motor is: Conservative. Non-conservative.

Conservation of Energy Non-conservative work changes mechanical energy. E1 = E2 K1 + U1 + Wnc = K2 + U2