.....is simply a notice or point of likeness and differences of two machines,purposed to use the power of gravity:
-Villard de Honnecourt's wheel (7 hammers but...a non-workable machine),intended to collect free gravity power.
-an impact test hammer (1 hammer and it works) ,a laboratory instrument that allows the delivery of calibrated gravity impact energies on test objects.
A hammer,usually is a tool with heavy head at end of handle,for beating,driving nails.
A hammer can be considered a particular pendular mass (rod-mass) also,but...it depends on you his usefulness (including to imagine a gravity motor...).
All the best! / Alex