A penny dropped from a skyscraper can kill
A penny is too light and flat to build dangerous momentum when falling. Air resistance brings it to terminal velocity within the first 15 meters of descent, and at that low speed it causes little more than a sting if it strikes someone.
What we know
The myth rests on a misunderstanding of terminal velocity. In a vacuum, a penny dropped from 440 meters (the height of the Empire State Building) would accelerate to roughly 335 km/h. In reality, the coin falls through air. Because a penny is very light and has a large surface area relative to its mass, aerodynamic drag increases rapidly and balances gravity within about 15 meters of descent. After that point the penny falls at a constant terminal velocity of approximately 40 to 65 km/h, fluttering like a leaf.
Physicist Louis Bloomfield at the University of Virginia conducted controlled experiments, firing pennies at terminal velocity into ballistics gel and concrete. The results, also replicated on the television program MythBusters, showed the penny could not penetrate the gel or chip concrete. At most it would sting or leave a small welt on exposed skin. The coin's flat shape also causes it to spin and tumble erratically, dissipating energy further.
In contrast, a narrow, dense, aerodynamically stable object such as a ballpoint pen dropped from the same height could reach speeds exceeding 320 km/h and embed itself in wood or seriously injure a person. The danger from dropped objects is real, but it is driven by density and aerodynamics, not simple height.
Common claims
- A penny dropped from the Empire State Building would kill a pedestrianFalse - terminal velocity too low
- The penny would accelerate all the way to the groundFalse - air resistance creates terminal velocity after about 15 meters
- Dropped objects from tall buildings can be dangerousPartially true - only for dense, aerodynamic objects like pens or bolts