City-slickers: Have you ever been afraid that a penny thrown off the sky of something like nearby skyscrapers could kill you at any moment?
You might ease – at least on this though. Transforming a penny into a dangerous weapon is incredibly tough, and flinging it over the Empire State Building’s barricades won’t do the trick. A penny is too small, flat, and cushioned by too much air to create a torpedo even from that height.
Eventually, like a wing, it would move to the ground. It would feel like being slapped all across the face — “but not very hard,” according to Louis Bloomfield, a physicist at the University of Virginia. And he should be aware. He recently recreated the fall of pennies from skyscrapers using aerodynamics and helium balloons. It didn’t hurt when he was struck with experimental coins. Bloomfield told Life’s Little Mysteries, “I guess one bounced off my face once.”
People wrongly believe that a falling penny, when subjected to gravity, will accelerate throughout its entire fall, reaching dizzying speeds by the time it reaches the ground. This would happen if New York City was evacuated — that is if all the air was eliminated and the penny was thrown off the Empire State Building into a vacuum — but as things stand, collisions with air molecules slow down the descent of pennies. Air resistance, sometimes known as “drag force,” opposes the penny’s downward motion, counteracting gravity.
The more quickly the penny descends, the more air resistance it encounters, until the drag force equals and opposes the downwards gravitational attraction at a certain maximum velocity. The penny no longer accelerates once the two forces are matched. Instead, it falls to the ground at a constant speed, known as the terminal velocity.
Pennies are flat, so they encounter a lot of air resistance, and they’re light, so they don’t require much drag to compensate for their weight. When thrown from a building, pennies reach terminal velocity after only 50 feet (15 metres) of descent. They flutter to the ground at a meagre 25 mph (40 kph) after that, according to Bloomfield.
A falling penny would accelerate to 208 mph (335 kph) by the time it hit the ground if there was no air (or your head). It could injure your head at that speed, but it wouldn’t bore through.
Bloomfield explained, “A penny is very much a small nothing.” “It’s not a particularly compact object. It doesn’t resonate with you.”
But grasp off on wearing your protective headgear. The direct threat is falling ballpoint pens. It would be lethal if someone casually tossed one of them from the Empire State Building. Pens can either spin and flip or shoot down like an arrow, depending on their design. “It might fall at 200 mph,” Bloomfield stated in the latter case. “It will strike a limited region with a lot of momentum when it hits. The stairway will be chipped. It can pierce a wooden board. You don’t want it to collide with your head.”
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