Question

why a black smith uses a heavier hammer that the one used by a gold smith?

Answer 1

A blacksmith deals with hard metals such as iron and steel,so large force is needed. Force is directly proportional to the mass of the hammerhead. Hence a heavy hammer is used by a blacksmith.
Whereas a goldsmith deals with soft metals such as gold,silver.So a smaller force is sufficient here and therefore a light hammer is used.