My hand grips the handle near its end. Notice the how the end of the handle moves in a nearly straight line during the early part of the blow, contrary to the intuitive sense that a hammer is "swung" in a sort of roundhouse blow.
The hammer head's path is also nearly linear over more than a third of its flight. What are the forces that accelerate the head along this path? The center of gravity of the hammer is only slightly back of the center of gravity of the head and the only significant forces are those exerted by my hand and by gravity.
See that during rebound I continue to pull the hammer toward myself, the direction of the force that was radial as the hammer descended. Then I push forward at the same time that I begin to lift the hammer.
(Note that the positions plotted here don't represent equal time intervals so you can't estimate acceleration from the distances between them.)
Updated 9 May 2000 -- Mike Spencer