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 energyCh.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)