13-2 Internal energy(p]ae) in a system of particles 1 Consider an ice skater she starts at rest and then extend her arm to push herself away from the ailing at the edge of a skating rink(溜冰场) + Use work-energy railing relationship to analyze g △+△U=Wgxr F The system chosen Ice only include the skater g →△U=0 Ch 13 Energy III: Conservation of energy
Ch.13 Energy III: Conservation of energy 13-2 Internal energy (内能) in a system of particles 1. Consider an ice skater. She starts at rest and then extend her arm to push herself away from the railing at the edge of a skating rink（溜冰场）. •••• ice railin g Use work-energy relationship to analyze: K + U =Wext The system chosen only include the skater Mg N F U = 0
WE=O: W N+MG 0 Wext=0 今△K=0??? in disagreement with our observation that she accelerates away from the railing Where does the skater's kinetic energy come from? Ch 13 Energy III: Conservation of energy
Ch.13 Energy III: Conservation of energy WF=0; WN+MG=0; Wext=0 K = 0 ??? in disagreement with our observation that she accelerates away from the railing. Where does the skater’s kinetic energy come from?
The problem comes from The skater can not be regarded as a mass point but a system of particles For a system of particles it can store one kind of energy called"internal energy:△K+△U=Wxt △K+△U+△Emt=Wa(13-2 It is the internal energy that becomes the skater's kinetic energy. Ch 13 Energy III: Conservation of energy
Ch.13 Energy III: Conservation of energy For a system of particles, it can store one kind of energy called “internal energy”. The problem comes from: The skater can not be regarded as a mass point, but a system of particles. K + U =Wext It is the internal energy that becomes the skater’s kinetic energy. K + U + Eint = Wext (13-2)