  The response of a trapped Bose-Einstein condensed gas to a density
perturbation generated by a two-photon Bragg pulse is investigated by solving
the time-dependent Gross-Pitaevskii equation. We calculate the total momentum
imparted to the condensate as a function of both the time duration of the pulse
and the frequency difference of the two laser beams. The role of the dynamic
response function in characterizing the time evolution of the system is pointed
out, with special emphasis to the phonon regime. Numerical simulations are
compared with the predictions of local density approximation. The relevance of
our results for the interpretation of current experiments is also discussed.
