Its potential energy: dw,-Gdv dx ->G=u"p dW, pu .d-pdsin UIts potential energy： 2 2 1        dx dy dWp GdV  2 G  u 2 2 2 1        dx dy dW u p  sin ( ) u x t u A dx dy      sin ( ) 2 1 2 2 2 u x dW dVA t  p     