Could a penny falling from a building really be lethal?

Imagine a tourist on the observation deck of the Empire State Building, reaching into his pocket for a penny. He moves to the edge and lets the coin fall past the protective barrier, plummeting 1,400 feet (427 meters) toward the ground below. The coin strikes the head of an unsuspecting passerby, resulting in their death — turning what seemed like a harmless prank into a tragic event.

Although this event hasn’t actually occurred, it’s become such a widely circulated tale that it’s now considered an urban legend. And, like many legends, the "drop a penny, kill a person" story contains a kernel of truth.

A penny weighing only 2.5 grams and measuring just 1.52 millimeters thick would not be able to inflict fatal damage if dropped from a great height. Its small size, with a diameter of 19.05 millimeters (roughly three-quarters of an inch), combined with its flat design, causes it to flutter like a leaf rather than descend in a straight, lethal path [source: United States Mint].

In this situation, velocity plays a key role. Although gravity pulls the penny downward, it doesn't accelerate continuously. The penny faces air resistance, which gradually opposes its fall and reduces its speed.

As the penny falls faster, it encounters more air resistance. Eventually, the forces of drag and gravity balance out, leading the penny to reach terminal velocity. After falling about 50 feet (15 meters), it reaches a constant speed of 25 mph (40 kph), a velocity that won't turn the coin into a deadly projectile, though it could still cause discomfort [sources: Wolchover, NASA].

As for the partial truth in the story? Other objects, when dropped from a significant height, could indeed harm those below. For instance, a roll of 50 pennies weighing 125 grams (about a quarter of a pound) could generate an impact speed of 20 mph (32 kph) if dropped from 1,400 feet (427 meters). This could lead to a fatal head injury. Although this calculation doesn't factor in air resistance, which depends on the object's shape and the air's density, an object of this mass could potentially be deadly despite some drag.

Here’s the broader perspective on protecting your head: For an object to become deadly when dropped, it must have sufficient mass and fall free from any updrafts caused by nearby buildings. So, if you're walking on a city street and spot a penny falling, it's safe to wait for it to land and pick it up. If it lands heads up, you might just be lucky!