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The gravitational-wave energy flux produced during the head-on infall andcollision of two compact objects is calculated using two approaches: (i) apost-Newtonian method, carried to second post-Newtonian order beyond thequadrupole formula, valid for systems of arbitrary masses; and (ii) ablack-hole perturbation method, valid for a test-body falling radially toward ablack hole. In the test-body case, the methods are compared. The post-Newtonianmethod is shown to converge to the ``exact'' perturbation result more slowlythan expected {\it a priori\/}. A surprisingly good approximation to the energyradiated during the infall phase, as calculated by perturbation theory, isfound to be given by a Newtonian, or quadrupole, approximation combined withthe exact test-body equations of motion in the Schwarzschild spacetime.Comment: ReVTeX, 18 pages, 10 figures coming separately in .uu executable  forma

Head-on collision of compact objects in general relativity: Comparison of post-Newtonian and perturbation approaches