Forces acting on a non-stationary medium in the presence of a high-fre
quency electrostatic field include the ponderomotive force, the dissip
ative force (taking into account the slow non-stationarity of the syst
em),and the force due to the full change of the field momentum. For th
e fields of normal modes (propagating waves), only the ponderomotive f
orce acts on the medium.