  Dynamics of relativistic outflows along the rotation axis of a Kerr black
hole is investigated using a simple model that takes into account the
relativistic tidal force of the central source as well as the Lorentz force due
to the large-scale electromagnetic field which is assumed to be present in the
ambient medium. The evolution of the speed of the flow relative to the ambient
medium is studied. In the force-free case, the resulting equation of motion
predicts rapid deceleration of the initial flow and an asymptotic relative
speed with a Lorentz factor of 2^1/2. In the presence of the Lorentz force, the
long-term relative speed of the clump tends to the ambient electrical drift
speed.
