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Re: [Phys-l] Final velocity of bullets



Now that we are finally trying to deal with the type of physics intended
by the original post, I can tell you the results of some experiments my
friends and I did when I was in grad school. We were not working with
the bullet fired in the air problem; rather, we were working with the
penny dropped off the Empire State Building problem. It is still
commonly stated that if a penny were dropped from the Empire State
Building and it were to land on a person's head, it would kill the
person. Sometimes the myth even states the penny will penetrate clear
through the person.

The bullet and penny problem are essentially the same... What is the
likely terminal velocity of a falling penny (or bullet).

One source of this penny claim is when someone with a little physics
knowledge calculates the velocity at which a penny will land if it has
acceleration g all the way from release to landing. Of course this is
not true because of air friction. Some who try to include air friction
assume the penny falls edgewise because that presents the least air
friction and they assume the penny will assume that orientation as it
falls.

When I was in grad school we didn't have the Internet, let alone Google,
so it was easier to do a few experiments than to see if anyone else had
done it. Sometimes I wish we were back in those days... Quit trying to
analyze the problem to death... Just go out and make some measurements.

We were never able to get a penny to fall edgewise. It very quickly
tumbles, and it tumbles rapidly. It's terminal velocity is very slow,
about 15 mph, roughly 20 ft/s, roughly 6 m/s. I can ride my bike that
fast. At that time a penny was copper. Today with copper-coated zinc
pennies I presume the terminal velocity is lower yet. Anyway, a penny
dropped from a high place is not going to kill someone.

Of course if someone were going to drop a penny or a bullet on my head,
I would prefer the penny. I assume the bullet will tumble, but it is
more dense and would probably have a higher terminal velocity than the
penny. The higher density and higher velocity should produce more
damage when it hits. Beyond that I haven't done any experiments with
falling bullets, so I won't speculate on the terminal velocity and the
amount of damage it will do, but I invite some of you to do some
experiments. You can't sit in your comfy desk chair and type; you have
to go someplace and set up the experiment. I would hope you would find
that fun. Of course the Myth Busters guys do just that. The problem is
they are not very good experimentalists, but at least they try it.

By the way, if you Google penny + "empire state building" you'll get
about 100 kilohits.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu