How high does a building have to be for a penny dropped from the top to kill a person on the ground?
Category: Physics Published: December 10, 2012
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Dropped pennies cannot kill people, no matter how high you drop them from. The reason is that air slows things down. Falling objects reach a maximum speed in air, called the terminal velocity, and can't go any faster. The less dense an object, the lower its terminal velocity. Think about letting go of a balloon from a high building. It's not going to hurt any one. The terminal velocity of a penny is 25-70 mph depending on how it tumbles as it falls, according to Jim Carson. While this seems high, a penny is so light that it can't do much damage at this speed. What if you hold your hand out the window of your car on the highway and a bit of loose gravel flings up and hits you on the arm? It would sting, but it wouldn't kill you. That's what a penny dropped from any height would feel like. Now dropping bowling balls and wrenches from a high perch is much more dangerous. Please don't try it.