Choose a segment infinite small dy Sdx dm pSdx Its kinetic energy: dw=dmv "V= y=-Aosin o(1- dw=(pdv)Ao'sin'o(t- Choose a segment infinite small dV  Sdx dm  Sdx Its kinetic energy： 2 2 1 dW dmv k  sin ( ) u x A t t y v          ( ) sin ( ) 2 1 2 2 2 u x dW dV A t  k     