If I can remember back twenty odd (at least some were) years to my
college Physics, a great deal of time was spent discussing force,
velocity, and acceleration. As it was put simply to this Freshman,
acceleration is a change in velocity and can only come about through
a force applied to the subject over some period of time.
In our example here, the force is the pianist pushing down on the key
for the time it takes the hammer to go through let-off. After that
point, the hammer can no longer be under any force and cannot,
therefore, be accelerating.
That said, a very firm blow in a grand can make the hammer shank bend,
or "whip". That is, the knuckle end of the shank moves sooner and
faster than the hammer end, where there is a relatively large mass.
In this case, it may be possible for the hammer to accelerate after
let-off, as it unbends. Has anyone ever seen an ultra slow motion view
of this? I am curious as to whether the shank straightens before impact
or whether this is significant to tone. By the way, in some pianos,
a firm blow can make the key whip, also.