The hammer is most certainly _not_ accelerating after the jack is
disconnected from the hammer butt.  _Any_ system that accelerates _must_
have energy put into it and the jack is the only energy input the
hammer has.  In fact, gravity is taking its toll the instant the jack
stops imparting energy to the hammer and the hammer has to be
de-accelerating at that point.

The strings are the most dramatic force stopping the hammer, however,
gravity makes no exceptions to its irresistible force.  The three things
that determine tone are velocity, mass and firmness of the hammer felt
(not considering the myriad of variables the strings control).

Jason Stahl