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Choice of System andthe Energy EquationBruce SherwoodRuth ChabayDepartment of PhysicsNorth Carolina State UniversityNC STATE UNIVERSITYThis 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 energyintextparticles WWE extintparticles )( WWE 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.extparticlessys WUEE Well consider situations where there is negligible Q (heat; microscopic work)Pair-wise interaction energy2 particles: 1 pair3 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:MoonEarth,MoonSun,EarthSun, UUU System: falling rock aloneextsys WE mghK Interpretation: The Earth, which is external to the chosen system, does positive work on the system, increasing the kinetic energy of the rock.hSystem: Earth plus falling rockextsys WE 0 UKInterpretation: 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.0)( mghKhEarthThe 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.mghmghK )(extWUK Change in potential energy isassociated with change of shapeEarthEarthThere is no change of shape of the rock alone.A more complicated exampleBarbell alone: mghFhK FhUK Barbell + Earth:0int EUKBarbell + Earth + woman:Matter & InteractionsRuth Chabay & Bruce SherwoodJohn Wiley & Sons, 2002http://www4.ncsu.edu/~rwchabay/mi