Potential energy by spring element Es =Jfdu=(kut)idr k[u sin(t)]Lou cos(t-)]dt=0 Energy by inertia force Ex -fdu-(miyidr -m[-oiusin(t-)][ou cos(-)]di=0 2π/0 目 土本程李悦 • Potential energy by spring element       2 / 0 2 / 0 0 0 sin cos 0 E f du ku udt S s k u t u t dt                           • Energy by inertia force       2 / 0 2 / 2 0 0 0 sin cos 0 E f du mu udt K I m u t u t dt                          