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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Choice of System andthe Energy Equation,Bruce SherwoodRuth Chabay,Department of Physics,North Carolina State University,NC STATE,UNIVERSITY,This project was funded in part by the National Science Foundation(grants,DUE-0320608 and DUE-0237132,).Opinions expressed are those of the authors,and not necessarily those of the Foundation.,Review of potential energy,Interpretation:Potential energy is interaction energy of pairs of particles,the internal work done by all pairs of particles in a system on each other.,Well consider situations where there is negligible,Q,(heat;microscopic work),Pair-wise interaction energy,2 particles:1 pair,3 particles:3 pairs(12,23,13),4 particles:6 pairs(12,23,34,13,14,24),For example,in the Sun+Earth+Moon system there are 3 terms in the potential energy:,System:falling rock alone,Interpretation:The Earth,which is external to the chosen system,does positive work on the system,increasing the kinetic energy of the rock.,h,System:Earth plus falling rock,Interpretation:No external work is done on the Earth-rock system.The kinetic energy of the rock increases.The pair-wise interaction energy(potential energy)decreases.,h,Earth,The,rock,has no potential energy,(Mis)interpretation leads to double counting.Potential energy is associated with pair-wise interactions inside a system.The rock alone has no potential energy.Rather,external work is done on the rock by the Earth.,Change in potential energy isassociated with change of shape,Earth,Earth,There is no change of shape of the rock alone.,A more complicated example,Barbell alone:,Barbell+Earth:,Barbell+Earth+woman:,Matter&Interactions,Ruth Chabay&Bruce Sherwood,John Wiley&Sons,2002,:/www4.ncsu.edu/rwchabay/mi,
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