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Some Common Myths Thought to be True - Myth 93
Myth 93: Penny Dropped from Empires State Building Can Kill
City-slickers: Have you ever worried that, at any moment, you could be struck
dead by a penny flung off the roof of a nearby skyscraper?
You can rest easy - on that score, at least. In fact, it's extremely difficult
to turn a penny into a lethal weapon, and hurling it over the barricades at the
top of the Empire State Building won't get the job done. Even from that height,
a penny is too small and flat, and cushioned by too much air, to become a
missile.
Instead, it would flutter to the ground like a leaf. If it did strike you, it
would feel like being flicked in the forehead - "but not even very hard," said
Louis Bloomfield, a physicist at the University of Virginia. And he should
know. He recently used wind tunnels and helium balloons to replicate the fall
of pennies from skyscrapers. When experimental pennies struck him, it didn't
hurt. "I think one bounced off my face once," Bloomfield told Life's Little
Mysteries.
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People often assume that a falling penny, subject to the force of gravity, will
accelerate for the entirety of its fall, achieving breakneck speeds by the time
it reaches the ground. This would indeed happen if New York City was evacuated
- that is, if all the air were removed and the penny was tossed off the Empire
State Building into a vacuum - but as things are, collisions with air molecules
slow falling pennies down. Called a "drag force," air resistance opposes the
penny's downward motion, counteracting the force of gravity.
The faster the penny falls, the greater the air resistance it experiences, and
so at a certain maximum velocity of the penny, the drag force becomes equal and
opposite to the downward gravitational force. With the two forces balanced, the
penny no longer accelerates. Instead it falls at a constant speed, called the
terminal velocity, all the way to the ground.
Pennies are flat, so they experience a lot of air resistance, and they are
light, so it doesn't take much drag to counteract their weight. Thus, if hurled
off a skyscraper, pennies achieve their terminal velocity after only about 50
feet (15 meters) of descent. After that point, they flutter to the ground at a
measly 25 mph (40 kph), Bloomfield said.
But don't take off that protective head gear just yet. Falling ballpoint pens
are the real danger. If someone nonchalantly tossed one of those off the top of
the Empire State Building, it could kill. Depending on their design, pens will
either spin and flutter, or shoot down like an arrow. In the latter case, "it
might well come down at 200 mph," Bloomfield said. "When it hits, it will hit a
small area with a lot of momentum. It will chip the sidewalk. It could punch
into a wooden board. You wouldn't want it to hit your head."
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