  We derive the force exerted in the background plasma by an arbitrary
distribution of non interacting quasi-particles, corresponding to either
collective excitations of the plasma (plasmons, phonons) or em dressed
particles (photons, neutrinos). Our approach is based on the effective
Hamiltonian describing the quasi-classical dynamics of the individual particles
in the presence of a background medium. We recover the usual results for the
relativistic ponderomotive force of a photon gas, and we derive the force, due
to weak interactions, exerted by the electron-neutrinos in a background medium
containing electrons, positrons and neutrons with arbitrary distribution
functions. Generalization to other background species and other neutrino
flavors is also discussed.
