  We solve the Gross-Pitaevskii equation to study energy transfer from an
oscillating `object' to a trapped Bose-Einstein condensate. Two regimes are
found: for object velocities below a critical value, energy is transferred by
excitation of phonons at the motion extrema; while above the critical velocity,
energy transfer is via vortex formation. The second regime corresponds to
significantly enhanced heating, in agreement with a recent experiment.
