Why a penny stops speeding up
A falling coin does not accelerate forever. As it picks up speed, it has to shove air out of the way more forcefully, and that air pushes back. Eventually, the upward drag balances the downward pull of gravity. The coin then falls at a steady speed called terminal velocity.
A penny’s flat, light shape gives the air plenty to push against relative to the coin’s weight. It may wobble or tumble as it falls, changing its orientation, but it still cannot build up the speed a dense, streamlined object would reach over the same distance.
Why the impact is usually not deadly
The danger of a falling object depends on more than how fast it is moving: its mass matters too. A penny has very little mass, so even at its terminal velocity it carries limited momentum and impact energy. A typical strike is unlikely to break through the skull, which is why the familiar image of a dropped coin drilling through someone’s head is misleading.
That does not make a falling penny harmless. It can sting, and a blow to an eye can cause serious injury because the eye is delicate and exposed. The same small object that is unlikely to pierce bone can still do real damage in the wrong place—so the physics is a reason not to panic, not a reason to drop coins on people.
