We have measured the drag and inertial forces on wire resonators in superfluid He-4 at millikelvin temperatures. At low velocities the behavior is dominated by the intrinsic properties of the resonators which can be measured in vacuum. On increasing velocity we see a sharp transition corresponding to the sudden appearance of drag and inertial forces arising from quantum vortex lines in the superfluid. We present detailed measurements of the superfluid drag and inertial forces as a function of the resonator velocity. We discuss results for two different wire diameters and for different resonant frequencies. We compare and contrast the observed behavior with that measured for cylinders in oscillating classical fluids.