A penny dropped from a skyscraper can kill
A penny dropped from a tall skyscraper cannot kill a person on the ground below. Its light weight and flat, irregular shape mean air resistance brings it to a low terminal velocity within roughly the first 15 meters of its fall, and at that speed it would cause at most a minor sting rather than any dangerous injury.
What we know
The claim that a penny dropped from the top of a very tall building, such as the Empire State Building, could strike and kill a pedestrian below by building up lethal momentum during its fall is a piece of popular physics folklore that, despite its wide circulation, does not survive an examination of how terminal velocity and air resistance actually work for a small, light, irregularly shaped object like a coin.
Terminal velocity is the maximum speed a falling object reaches when the upward force of air resistance acting against its motion becomes equal to the downward force of gravity pulling it down, at which point the object stops accelerating and falls at a constant speed for the remainder of its descent, regardless of how much additional height remains above it. Terminal velocity depends heavily on an object's mass relative to its cross-sectional area and shape: objects that are light for their surface area, and especially those with an irregular, non-aerodynamic, tumbling shape, reach a comparatively low terminal velocity very quickly, since even a modest air resistance force is enough to balance out their relatively small gravitational pull.
A penny is both very light, weighing roughly 2.5 grams for a modern U.S. penny, and has a relatively large, flat surface area for that weight, and importantly, pennies do not fall in a stable, straight, aerodynamic orientation; they tumble, flutter, and spin somewhat unpredictably as they fall, which further increases the effective air resistance acting on them throughout the descent. Physics analyses and demonstrations of this scenario, including some that have used high-speed photography and wind tunnel testing to directly measure falling coin behavior, have found that a tumbling penny reaches terminal velocity at a fairly low speed, generally estimated in the range of roughly 25 to 50 kilometers per hour, within only about the first 15 meters of its fall, meaning that for the remaining several hundred meters of a drop from a skyscraper's height, the penny is not accelerating or gaining any additional speed or force whatsoever.
At the terminal velocity a tumbling penny actually reaches, the kinetic energy delivered upon impact is quite modest, and physics educators and organizations, including the popular science myth-testing television program MythBusters, have directly tested this scenario using both calculation and physical simulation, consistently finding that a penny falling from a great height and striking a person would produce, at most, a minor sting or very slight surface mark comparable to being struck by a light flick or a small pebble, and would not be capable of piercing skin, causing serious injury, or approaching anything close to a lethal impact under any realistic real-world falling scenario.
This penny myth serves as a frequently used illustrative example in introductory physics education specifically because it demonstrates, in an intuitively engaging way, the important and sometimes counterintuitive concept that falling objects do not simply keep accelerating indefinitely the further they fall, and that an object's terminal velocity, determined by the interplay of its mass, shape, and air resistance, places a hard ceiling on how much kinetic energy it can ultimately deliver upon impact regardless of the total height of the fall, a principle that applies very differently, and far more dangerously, to heavier, more aerodynamically stable falling objects, which is why falling debris from height in the form of tools, bricks, or other heavier and more compact objects remains a genuine and well-documented safety hazard on construction sites, in clear contrast to the physically implausible penny scenario.
Common claims
- A penny dropped from a skyscraper can kill someone below.Not supported
- A falling penny reaches terminal velocity quickly due to its light, flat shape.Accurate
- Falling objects keep accelerating no matter how far they fall.Not supported
- Heavier, aerodynamically stable falling objects pose a genuine safety hazard.Accurate