You've heard it before: don't drop a penny off a skyscraper, because it could kill someone. It's a notion as strangely comical as it is disturbing. But the idea that a small coin could cause death upon impact seems plausible, right? If you think that, you're not alone - some physicists have even tested this theory by replicating the conditions of throwing a penny or two from the Empire State Building.
Most people would automatically assume that a penny tossed at such a height would be dangerous. However, the physical outcome of a penny falling from the sky and coming into contact with someone is pretty surprising.
So, the big question is: could a penny kill you? No, and it probably wouldn't even hurt you. But the physics of falling pennies would still make for fascinating conversation at your next awkward dinner party.
The Penny Is Ultimately Slowed Down During Its Fall
Anything falling from an immense height - say, the top of a skyscraper - is subject to the force of gravity. Because of this, you might assume that a falling penny would reach dangerous speeds. However, falling pennies are actually slowed down as they move, due to collisions with air molecules.
Air Resistance Builds The Faster The Penny Falls
The air resistance a penny experiences while falling is referred to as a “drag force.” This air resistance opposes the downward motion of the penny, and the air resistance builds the faster the penny falls. So, once the penny reaches the highest velocity it can possibly attain during its fall, the drag force counteracts gravitational forces.
The Penny Stops Accelerating Once Drag And Gravitational Forces Become Equal
Pennies experience quite a bit of air resistance because they are flat. The coins are also fairly lightweight, so it doesn’t take much drag force to counteract their weight against the force of gravity. Once the drag and gravitational forces become balanced, the penny no longer accelerates.
The Penny Is Still Moving Pretty Fast For A While
Once acceleration ceases, the penny falls at a constant speed all the way to the ground (or whatever or whomever it may run into beforehand). This constant speed is called the terminal velocity. Because pennies are lightweight and flat, they reach their terminal velocity after falling 50 feet. According to physicist Louis Bloomfield, the penny’s speed before reaching this velocity is documented to be around 208 miles per hour.