posted by Anonymous .
If you ask a friend to drive a small nail into a piece of wood placed on top of a pile of books on your head. Why doesn't this hurt you?
Well, I would want a pretty massive pile of books and a light nail.
Force times time = change in momentum.
The hammer delivers a big force for a very short time. That force on a very small nail may change its velocity quickly (big acceleration with big force because of small mass.) However the big force does not last long enough to change the speed of the pile of books much (big force, big mass, small acceleration.) Therefore the pile of books does not transmit much force on to your heard.
If you want a headache, use a sledgehammer, big 20 penny nail, and a copy of Time magazine.