  We consider the C-metric as a gravitational field configuration that
describes an accelerating black hole in the presence of a semi-infinite cosmic
string, along the accelerating direction. We adopt the expression for the
gravitational energy-momentum developed in the teleparallel equivalent of
general relativity (TEGR) and obtain a possible explanation for the
acceleration of the black hole. The gravitational energy enclosed by surfaces
of constant radius around the black hole is evaluated, and in particular the
energy contained within the gravitational horizon is obtained. This energy
turns out to be proportional to the square root of the area of the horizon. We
find that the gravitational energy of the semi-infinite cosmic string is
negative and dominant for large values of the radius of integration. This
negative energy may explain the acceleration of the black hole, that moves
towards regions of lower gravitational energy along the string.
