content_image/373387.jpg content_image/373389.jpg content_image/373388.jpg
and hence 

<img src='content_image/373387.jpg'>

After substituting the equations (20), (21) into (3) and (4) and ignoring the polarization gradient, we have 

<img src='content_image/373389.jpg'>

The kinetic energy is equal to 

<img src='content_image/373388.jpg'>

The work done is described by the equation (12). 

Applying the Hamilton’s principle similar to previous section, we obtain the equations of 

