Monday, July 28, 2014

Penny and Empire State Building

You may have heard that dropping a penny (or any other very small object) from the Empire State Building (or any other tall building), that the penny will kill a person if it hits their head.  The idea is that the penny is falling so fast that despite its small size, it will have enough velocity and energy to crack and penetrate a person's skull.  Fortunately for pedestrians near tall buildings, this is simply not true.

Why not?  Air.  The air the penny falls through produces an upward drag force (friction).  As the penny falls, this drag force increases until it matches the force of gravity downward.  When these two forces equal, the penny reaches a terminal velocity and simply can't fall down any faster.  The terminal velocity of a penny is 30-50 mph.  Even at the upper end of that range, given the small mass of the penny, there simply isn't enough energy to crack a person's skull.  A penny strike may sting, but it won't kill a pedestrian on the sidewalk.

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