# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Kelsey Tanner on 07/06/2008 12:42:42

Title: Why would a penny, dropped from the Empire State building, be lethal to anyone on the ground?
Post by: Kelsey Tanner on 07/06/2008 12:42:42
Kelsey Tanner asked the Naked Scientists:

I have a question about an urban legend popular here in the States. Why in the name of Abraham Lincoln would a penny dropped from the empire state building be lethal if a person stood between it and the ground? In other words, why do falling objects accelerate if gravity remains constant and, according to Isaac Newton (whom I assume to be a reliable source,) "An object in motion will not change its velocity until a net force acts upon it."

LOVE THE SHOW!!!! (http://www.thenakedscientists.com/HTML/podcasts/)

Kelsey Tanner
Nashville, Tennessee

What do you think?
Title: Why would a penny, dropped from the Empire State building, be lethal to anyone on the ground?
Post by: chris on 07/06/2008 14:23:47
I guess we'll need to know how heavy a penny is, what its terminal velocity will be and therefore whether we think it's capable of doing serious injury or not.

What I can say, subjectively, is that even objects falling from the sky and accelerated solely by gravity can cause serious or fatal injuries. For instance after the first Gulf war, when the Iraqi forces left Kuwait, at least 8 people were reported to have been killed by falling bullets after jubilant citizens fired guns into the air. So small, falling projectiles can be seriously bad for your health.

However, some bullets are streamlined and designed to travel through the air with minimal friction, so one would expect them to have a high terminal velocity.

A penny, on the other hand, presents a large surface area in one dimension and a smaller surface area in the other. One would expect it might therefore tumble or twist as it falls and this would probably slow it down. As a result, despite weighing the same (or possibly more) than a bullet a penny may well travel more slowly. Consequently it's kinetic energy (and hence the energy of the impact) will be considerably lower because the formula for kinetic energy is 1/2mv^2 - i.e. proportional to the square of the velocity.

Hopefully now someone with some numbers can flesh out these bones for us to provide an objective answer.

Chris
Title: Why would a penny, dropped from the Empire State building, be lethal to anyone on the ground?
Post by: Bored chemist on 07/06/2008 17:25:27
Someone has done the experiment.
http://en.wikipedia.org/wiki/MythBusters_(season_1)#Penny_Drop
Title: Why would a penny, dropped from the Empire State building, be lethal to anyone on the ground?
Post by: syhprum on 17/06/2008 14:00:56
If this is meant to be a serious discussion of the 'Modus operandi' of these vascular dilators it should of course be in a medical section of the group but I fear it is just a feeble attempt at advertising.

It appears the post to which my reply refers has already been removed!
Title: Why would a penny, dropped from the Empire State building, be lethal to anyone on the ground?
Post by: Flyberius on 20/06/2008 10:01:55
No.  Unless you got it in an eye, or an exposed area of brain.  I do recall on Mythbusters (highly scientific show, lol) them cracking a skull with a coin.  When dropping it failed they shot a penny at it.  Slightly macabre.