A tourist standing on the observation deck at the Empire State Building reaches into his pocket and pulls out a penny. He steps to the edge and tosses the coin over the protective fencing, where it falls some 1,400 feet (427 meters) toward the sidewalk below. The coin hits the skull of an unwitting pedestrian, killing the person — and turning a seemingly harmless prank into a deadly act.
While this hasn't actually happened, it's a scenario so often repeated that it's reached urban legend status. And, like a lot of urban legends, the "drop a penny, kill a person" story has a grain of truth at its core.
At just 2.5 grams and 1.52 millimeters thick, a penny flung off a tall building isn't likely to kill someone walking below. Plus, at 19.05 millimeters (three-quarters of an inch) across, a penny's flat shape will cause it to flutter as it falls, making its trajectory more like a falling leaf than a potentially lethal arrow [source: United States Mint].
Velocity also comes into play in this scenario. While a penny will be subject to gravity as it falls, it won't experience a continual acceleration. Instead, it will encounter the drag of air resistance, which will counteract its acceleration.
The faster a penny falls, the more air resistance it meets. At a certain point during its fall, the forces of drag and gravity become balanced, and the penny begins to travel at a constant rate. This is known as terminal velocity. A penny will reach terminal velocity after falling about 50 feet (15 meters), then will travel at 25 mph (40 kph) until it reaches the ground. That rate won't turn the single coin into a killer, though it could cause pain [sources: Wolchover, NASA].
As for the grain of truth we mentioned? There are other objects that, if dropped from a great height, could harm a passer-by below. Take a roll of 50 pennies. At 125 grams – a quarter of a pound – a fall of 1,400 feet (427 meters) would generate an impact speed of 20 mph (32 kph), which could result in a deadly blow to the head. While these calculations don't take into account air resistance, which can vary depending on object shape and air density, an object of this weight could prove to be a killer, even with a fair amount of drag.
Here's the big picture for your head's safety: For an object tossed from a building to become deadly, it needs to have sufficient mass. It also needs to fall free of updrafts caused by neighboring structures. So, if you're walking down a city sidewalk and spy a penny plummeting to earth, you could wait for it to land and pick it up. As long as it's heads up, it might be lucky!