同济大学：《大学物理》课程教学资源（电子教案，英文版）第九章 机械波（中）

Mechanical waves II)

Mechanical waves (II)

agenda today 1.Energy of harmonic waves 2.Interference of mechanical waves 3.Standing waves

Agenda today 1. Energy of harmonic waves 2. Interference of mechanical waves 3. Standing waves

Energy of harmonic waves Energy travels when waves travel Average energy of medium When transverse waves travel in some elastic medium:

Energy of harmonic waves Energy travels when waves travel When transverse waves travel in some elastic medium： Average energy of medium

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     

Its potential energy: dw,-Gdv dx ->G=u"p dW, pu .d-pdsin U

Its 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     