Have you heard the one about the penny killing that guy under the Empire State Building? It's really skull-crackingly hilarious, isn't it?It's a popular urban legend that a penny dropped from a tall building can not just cause grievous injuries, but can outright kill the unfortunate recipient of this ironic windfall. Many actually believe that the killer penny can not only kill people, but can seriously damage cars, and can cause cracks in the pavement.
A penny is the lightest coin in any denomination, but many people think that the height of skyscrapers or tall structures would enable the admittedly light penny to reach a fatal velocity. So, would making a wish and flinging a penny from the top of a skyscraper be the cause of tragedy for some innocent fellow standing below?
Luckily, the low weight and flat, circular shape of a penny means that it can't penetrate the skull of a healthy human being. Babies are at risk from such objects, since their skulls are much weaker and undeveloped for the first few months, but even babies are not at mortal risk. Having said that, it would be wise to not try that out on a trial-and-error basis.
Why is a penny safe?
A penny is a flat, circular, light object. Its flat surface results in a high amount of drag. As opposed to bullets or missiles, pennies aren't shaped for aerodynamic optimization. Due to the peculiar wind patterns occurring around tall structures, objects dropped from the top face severe updrafts. This causes them to slow down, until their gravitational acceleration equals the force of the updraft. This stage is known as terminal velocity; objects falling at terminal velocity no longer accelerate due to gravitation, but merely fall at the constant terminal velocity. If the upward forces of the wind exceed gravitational acceleration, the penny will be thrown up, but this only happens in very windy conditions.
Due to the penny's weight, it reaches terminal velocity fairly quickly―at a mere 25 mph. It is so light, in fact, that it would not be fatal even if it was dropped in vacuum, thus preventing retardation caused by the updrafts. In vacuum, a penny would reach approx. 208 mph. An impact at this speed would certainly cause severe injuries, but would probably not kill. For comparison, most bullets are shot at more than five times that speed. If you are unfortunate enough to be hit by the edge of a penny on weak areas of the face such as eyes and the nose (which is statistically impossible) in vacuum, the aftereffects of the injuries may cause complications that lead to death. But, as previously mentioned, this is a statistical impossibility, and if you are in vacuum, falling pennies are the least of your worries!
Physicist Louis Bloomfield performed a series of experiments about this very urban legend. He even caught some pennies on his own face, and reported no pain. In his own words,
A penny is pretty much a little nothing. It's not a very compact object. It doesn't drill into you very well!
How did the legend arise?
Objects falling from a tall building are inherently dangerous. While a penny is too light to be sufficiently forceful, objects such as nuts and bolts, and keys can be dangerous. That is why hard hats are mandatory during construction.
Objects either heavier than a penny or having a sharp tip can indeed be fatal, given suitable wind conditions. A ball-point pen, for instance, could prove fatal if it managed to land on the tip. Since pens necessarily have a sharp tip, all the downward force of their fall is concentrated in the single point, instead of being distributed across a flat surface like with pennies. If the pen rolls and hits you on its side, it would be much less traumatic, but if it merely rotates and remains fairly stable, its accidentally aerodynamic shape would cause it to attain a much higher terminal velocity than pennies, and can deliver a deadly punch.
All things considered, it's fairly safe to walk by skyscrapers without worrying about tragic windfalls.