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Date: Tue, 30 Apr 1996 09:24:33 -0700
From: Dave Wilton <dwilton@ix.netcom.com>
Subject: FAQ Documentation: Penny Falling

My summary: According to Dennis Thompson (dthompson@lerc.nasa.gov), safety testing at NASA's Zero Gravity Research Center in Cleveland concluded that coins falling from a height of 510' in air achieve a terminal velocity that is too low to seriously injure a person it strikes, much less bury itself in concrete. Other more aerodynamic objects, such as pens, can achieve a significantly higher velocity and can constitute a hazard to people below.

The original post:

From: dennis.thompson@lerc.nasa.gov (DeeTee)
Newsgroups: alt.folklore.urban
Subject: Re: death by penny falling from CN Tower
Date: 29 Apr 1996 14:26:40 GMT
Organization: NASA Lewis Research Center

It's about time I jumped in on this thread. Allow me to introduce myself, I am the facility manager for NASA's Zero Gravity Research Facility at the Lewis Research Center in Cleveland.

The Zero-G is a 510' shaft into the ground. It contains a vacuum chamber 20' in diameter in which we drop test vehicles in a free-fall condition. While the tests are falling the 432' drop distance, they are at extremely low gravity (less than 1/1,000,000 G). It is the lowest gravity man can attain easily, far lower than on orbit (1/10,000 - 1/100,000 G), although for only 5.18 seconds.

Our testing is done at vacuum, so there is no air resistance. In this testing mode, objects drop straight, with no tumbling, and no terminal velocity. In air, however, there is a tremendous difference. Unless the object is aerodynamically stable,it tumbles. Every object has a terminal velocity in air, and when it tumbles, the terminal velocity can be quite low.

Since there are times when people are working at the bottom of the shaft, safety is a great concern to us. Access to the top of the shaft is restricted at these times. For curiosity's sake,we have dropped things like coins, pencils, etc, to observe the potential for damage or injury.

Coins tumble rather well, and have a high surface/mass ratio anyway. Their terminal velocity seems very low as a result. When they hit, they bounce several feet high. While no one has volunteered to catch one, all agree that the impact might sting pretty good, but there wouldn't be any penetration or severe damage.

Pens are another matter. One particular solid metal Parker ball point dropped like an arrow! It steered to the side and missed our plywood target, but shattered on impact. I can't put any quantitative value on its speed, but it sure seemed to have the potential for serious hurt!

By the way, the speed of the test vehicles reaches 166 ft/sec, about 120 mph, so that is the upper limit of velocity for a 432' fall.

From: dennis.thompson@lerc.nasa.gov (NASA replies)
Newsgroups: alt.folklore.urban
Subject: Re: death by penny falling from CN Tower
Date: 1 May 1996 13:47:24 GMT

>Riiiiight. 1) I once called NASA, and they said you could not create
>zero g's on earth. Remember, gravity is the attraction of two bodies
>to each other: the earth and whatever is in this so-called chamber.

Well, if you called NASA, you didn't call the right person. Call ME, I am the facility manager of the Zero-G Facility, and we have been creating microgravity in this "so-called chamber" for 30 years - Dennis Thompson (216) 433-5485.

Also, check out our Web pages at
http://zeta.lerc.nasa.gov/facility/zero.htm and http://zeta.lerc.nasa.gov/facility/dtower.htm - sorry for the lack of pictures, got to correct that!

The "G" we are referring to is the gravitational attraction on earth, not the attraction between two small bodies. That force is so small as to be virtually unmeasureable. Remember, it takes the entire earth to make you weigh 200 lbs or whatever.

We are not doing away with gravity, just allowing it to be the ONLY force on an object (free fall). This is identical to the situation on orbit - the shuttle is FALLING. It has a constant acceleration towards the center of the earth, and a forward momentum such that the vector produces a curved path and it falls around the earth, right back to its starting point.

>2) I don't think humans would do very well in a vacuum environment.

They do quite nicely, thank you. They are called astronauts or cosmonauts. But we don't drop them in the Zero-G Facility due to the sudden stop (65 Gs). Most drop towers are at atmosphere anyway, they use drag shields to surround the test, which is not attached to the drag shield. The test falls the greater distance of the drop, and at the same time is falling within the drag shield, thus the only air resistance is the small velocity difference between the test and the shield. They are timed perfectly so that the two meet at the moment of impact in the decelerator.

>3) The only way to "create" zero-g's is to fly parabolic curves in
>special planes. Since you are in somewhat of a controlled stall, this
>is done over water, usually the Gulf of Mexico. That is how you train
>astronauts and how they filmed "Apollo 13".

The aircraft are not in a "controlled stall". They are flung at very high speed over the top of an arc called a "Keplarian Curve". The aircraft falls, just like the shuttle, just like our Zero-G test vehicles, just like you do when a car crests a hill, or a roller-coaster descends, or you ride the "Demon Drop' at an amusement park. That funny feeling in your stomach is low G. Believe it.

As to the aircraft's flights (my branch manages them also), they are flown in military restricted air space. One is over Selfridge AFB in Michigan, the other is over Wright Patterson AFB here in Ohio.

>But everyone knows that a penny dropped from the Empire State Building
>will kill someone 'cause they also put 6" holes in the sidewalks upon
>impact.

As I posted earlier, we drop coins and such to evaluate the safety aspects of them hitting someone at the bottom of our shaft. Coins tumble and quickly reach terminal velocity. After a 450 fall, they bounce nicely, but that's it. They don't bend or anything. We can't even find a mark on them.

Dennis Thompson,
Zero-G Facilty Manager,
Lewis Research Center,
Nasa,
USA,
Earth.