  Based on the dielectric formalism in the generalised random phase
approximation, we generalise the description of a Bose condensed gas to allow
for a relative velocity between the superfluid and normal fluid. In this model,
we determine the critical velocity dynamically as the transition point between
stable and unstable dynamics. Unlike the zero temperature case, at finite
temperature the relative critical velocity of a dilute Bose gas is lower than
the sound velocity. This result illustrates one relevant difference that exists
between a conserving and gapless approximation and other approaches.
