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Question:
Why do they say if you drop a penny off the Sears Tower, it could crush a car?


Replies:
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 the object.

Arthur Smith


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