dw.dw, Total energy: dwdW +dW,pdVAo'sin2 w(t-X) U the kinetic energy and potential energy of the infinitely small mass in the medium is the same. dw W= dy =PA0'sin2@(t-*) 7pfw'sm'og三d k p dW  dW Total energy： sin ( ) 2 2 2 u x dW dW dW dVA t  k  p      the kinetic energy and potential energy of the infinitely small mass in the medium is the same. sin ( ) 2 2 2 u x A t dV dW w       dt u x A t T w T sin ( ) 1 2 2 2 0      