Why do they say if you drop a penny off the Sears Tower, it could
crush a car?
Well, the penny speeds up as it falls - it gains "momentum",
and force comes from a change in momentum. A penny going
arbitrarily fast can produce an arbitrarily large force.
However, I think a penny probably could not crush a car
no matter how high it was dropped from because of air resistance -
this ensures that objects never go faster than a certain
"terminal velocity" that depends on the shape and mass of
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Update: June 2012