A common urban legend suggests that a penny dropped from a towering skyscraper could accelerate to such a dangerous speed that it becomes a lethal projectile upon impact, capable of causing serious injury or even death. This widespread misconception has led to questions about the true physics at play, but the reality is far less perilous than the myth suggests.
The Physics Behind the Myth
Objects falling through the atmosphere do not accelerate indefinitely; instead, they reach a maximum speed known as terminal velocity. As a penny drops, gravity pulls it downward, while air resistance pushes upward, counteracting acceleration. The faster the penny falls, the greater the air resistance it encounters. Eventually, these opposing forces balance, causing the penny to stop accelerating and maintain a constant speed.
A penny’s low mass and relatively large surface area contribute significantly to its interaction with air resistance. This combination means it reaches its terminal velocity quite rapidly, typically within about 15 to 50 feet of falling. After reaching this point, a penny will continue its descent at a relatively slow and constant speed, usually ranging between 25 and 50 miles per hour. This speed is far less than what many imagine, limited by air resistance.
The Real-World Impact
If a penny traveling at its terminal velocity were to strike a person, the impact would be surprisingly benign. It would feel like a light flick to the forehead or a mild tap, similar to a thrown pebble. This is because the penny’s momentum at terminal velocity is quite small. The minuscule force generated is insufficient to break the skin or cause serious injury.
The energy transferred by a falling penny is considerably less than what is required to inflict severe damage. For comparison, a typical raindrop, despite its small size, also reaches a terminal velocity, but its impact is barely noticeable due to its tiny mass. While a penny carries more energy than a raindrop, it still does not possess the kinetic energy needed to crack a skull or cause a lethal blow. Therefore, the notion of a penny becoming a deadly object when dropped from a great height is disproven by physics.