In a properly regulated piano the hammer cannot be accelerating when
it strikes the strings.  This is because if the let-off is adjusted
properly the impelling force is removed from the action shortly before
the string is struck.

The hammer is "thrown" at the string and I know of no possible way
for that hammer to accelerate after being "thrown" in such a manner.
In fact, in a standard grand action where the hammer is "thrown" upward
this might be even more obvious.

Hal Davis