Stephen Ashlee dispels a common misconception and alerts us to the real danger of killer pens
Money falling from the sky seems like everyone's dream doesn't it? But I bet opinions would change if it started falling in coins and not notes. It is a fairly popular notion that if someone were to drop a single penny off the very top of the Empire State Building, it would gain enough speed that it could kill someone at the base. But is there any truth to it? If the sky spontaneously started raining pennies, would they be deadly?
The simple answer is no. The worst you would get is a slight headache, but to be honest, a penny falling from that height would be going nowhere near fast enough to cause lasting damage. However, that’s not to say the myth is without scientific reasoning though.
If you drop any object, it’s going to be pulled towards the centre of the Earth by gravity, and all the time it’s doing so it’s accelerating. By doing a bit of simple maths, you can work out that dropping a penny, from rest, off the highest point of the Empire State building (443m), will eventually reach a speed of around 208mph. It sounds swift perhaps, but because a penny is so light, even this speed wouldn’t be fatal. It would probably damage your skull, but it would be survivable.
In reality though, the penny would never even reach this speed. The one thing that the myth fails to take into account is air resistance, or drag. As the penny is careering down the side of the building, it’s going to be constantly colliding with all the air molecules in its path. This produces a force which opposes gravity, labelled drag. As the penny gets faster and faster, this drag force gets bigger and bigger until a certain point where the drag force is large enough that it equals the gravity pulling the penny towards the Earth. At this point, the penny is going to stop accelerating and start falling at a constant speed. This is termed its terminal velocity.
Louis Bloomfield, a physicist at University of Virginia, decided to simulate falling pennies using wind tunnels and helium balloons. His findings were that it really only takes around 15m for the penny to reach terminal velocity. It’s such a short distance because of the flat disc shape of the penny, so instead of dropping straight down, it is more likely to flutter side to side like a leaf. Furthermore, the highest speed Bloomfield recorded was only around 25mph.
As described by Bloomfield, “I think one bounced off my face.”
Although you may no longer be worrying about falling pennies any time soon, the real danger you should look out for is ball-point pens; their aerodynamic shape gives them barely any drag. So if one falls from the height of the Empire State Building and hits you, you may end up with a bit more than a headache...
From Issue 13