  I discuss the scattering of phonons by a vortex moving with respect to a
superfluid condensate. This allows us to test the compatibility of the
scattering-theory derivation of the Iordanskii force with the galilean
invariance of the underlying fluid dynamics. In order to obtain the correct
result we must retain $O(v_s^2)$ terms in the sound-wave equation, and this
reinforces the interpretation, due to Volovik, of the Iordanskii force as an
analogue of the gravitational Bohm-Aharonov effect.
