[meteorite-list] Suspected Meteorite GoesThroughWindowinIllinois

From: MexicoDoug <MexicoDoug_at_meteoritecentral.com>
Date: Wed, 7 Mar 2007 19:03:42 -0600
Message-ID: <005e01c7611d$9f2079c0$abce5ec8_at_0019110394>

Chris wrote:
"In a steady 18 mph wind [which would give the metal a fall angle of 20
degrees], that equates to a horizontal ground speed of 18 mph. The heavier
the object, the longer it will take to reach that zero airspeed. So if this
thing was falling vertically, and then experienced a brief gust, it would
hardly be affected. On the other hand, if fell a mile in that wind, I'd
expect its ground speed to be close to the wind speed: it wouldn't be
falling vertically. This has little to do with aerodynamics."

OK Chris, I think I understand your assumptions and note your departure from
reliance on aerodynamics which it originally sounded like you were depending
upon for your assumptions. Now we are in the upstream/downstream textbook
canoe problem. Whether you originally depended on aerodynamics is debatable
though while my intuition had a problem with that, I think that is a harder
argument for me to critique. Just one comment and I will put some 'hard'
numbers to this which I invite you to pick apart if you can (or if not,
perhaps use them for your own thoughts).

"It is also why there can be no apparent relationship between the
orientation of a strewn field and the path of the meteor that produced it."

I think this is as poor an example as it is an interesting meteorite aside.
But I agree that the physics of the wind are at work for this case. My
objection: You are comparing the integrated effect of all winds in all
directions at all altitudes (for a significant distance) with local
atmospheric effects of a relatively unidirectional ''steady wind'.

Now for the numbers I promised:

This meteorthing has the following characteristics assuming it really is in
free fall as you believe (compared to the 60 mph suggested somewhere else,
that did not specify it was vt though that had to be the professor's
assumption):

Let me make a meteorthing terminal speed table showing how long it takes to
fall a mile:
Shield orientation: 38 m/s (86mph) 42 sec
"Average orientation" 47 m/s (105mph) 34 sec
Edge first orientation: 55 m/s (124mph) 29 sec
note: terminal speed is bound by 38 < vt < 55 m/s (86 < vt < 124 mph)

You mentioned: "On the other hand, if fell a mile in that wind, I'd expect
its ground speed to be close to the wind speed: it wouldn't be falling
vertically."

How long is a gust? Better yet, let's put some momentum 'flux' constraints
that the air imparts on the 'iron' given your 18 mph breeze that you believe
can alter the fall angle to 20 degrees from vertical. I am using the
average terminal velocity above, which you will see turns out actually to be
generous, I think.

The air imparts 1/103 (about one percent) of the momentum the meteorthing
has per second as it starts whittling the angle upward. To get the 20
degrees you claim, in the time frame you suggest, we theoretically need
34.2% (=sin(20)) of the momentum of the thing at minimum. So, in 36 seconds
will it be there? No. It will have just about traveled your mile, though.
Theoretically this is because as soon as it starts gaining angle, the
relative horizontal windspeed begins to equilibrate, and the momentum
transfer rate will actually go down by the inverse square of the ratio of
the relative windspeed to the 18 mph absolute windspeed. In other words,
you get the meteorthing half way up to the speed and the momentum flux is
already down to 25% of what it originally was. Now we're starting to fall a
couple miles and it gets harder.

However, it's worse: as this relative speed slows down and the momentum is
being added with all of the corresponding aerodynamic effects of thing
orientation frustrating attempts to increase the angle by doing a random
walk. Additionally, there will be the practical world problem that
windspeed is rarely steady even when we call it steady. A lull is an easy
push downhill to a smaller angle, while getting back is uphill again. Cross
drafts and anything outside of say a 60 degree cone are the same story. The
atmosphere is complex enough to make just a concept out of the words steady
wind.

What I think, is you will need a much stronger wind to keep this thing in
horizontal motion. I'll meet you half way and say we might get 10 degrees
out of the 18 mph wind. Or conversly a 36 mph 'steady' wind might work for
getting 20 degrees.

You mentioned:
"I've dropped similarly shaped rocks off of cliffs, and watched them fall.
They definitely don't and watched them fall. They definitely don't drop
straight, but look a lot like falling leaves."

I would love to do this on one of your nice Colorado cliffs, pay close
attention to you and compare notes and surely learning a great deal. Though
dropping rocks off a cliff bounded by a mountain is not all I would want to
do. I would bring a kite and a fishing rod and reel with high tech high
tensile low drag MICROfilament line. The kite,I would use to demonstrate
in the nice steady wind (helped perhaps by steadying mountain geography),
and how often it tugs, relaxes, and cross gusts and changes directions -
even though on the ground it feels pretty much the same. (and you are
talking about a 1609 meters fall, not less than my 100 meter kite line).
Then I would find a cliff with an 18mph appropriately directioned wind and
swap the kite for a sinker weight of nearly 1 pound (actually 402 gram)
meteorite shaped like a deck of cards. I would let it down 3 meters (10
feet) of line and see if it deflected one meter from vertical (=the 20
degree angle), which is over three feet. The I would toss the meteorite off
the cliff to land 100 meters below, and put a rare earth magnet on the end
of of my line and go fish for it starting vertically. I would then pick it
off the ground and see if during 30 seconds to one minute,the iron slowly
but surely creeped along a pendulum arc reasonably near the 34.2 meters to
one side - slowly, gracefully; and, surely? The is the small angle
approximation here, so I understand it would fall a little short. Then I
would like to interpret the results with you, and again compare notes.

Back to this:
"The object in question is quite small, and quite flat. It doesn't have to
be a very good wing to still generate some lift and produce an angled
descent (you'd probably use the term "crash" if an airplane landed at the
same angle as this chunk of metal). I've dropped similarly shaped rocks off
of cliffs, and watched them fall. They definitely don't drop
straight, but look a lot like falling leaves."

Now you are back to aerodynamics. So I am not sure if you are arguing case
one of air movement, or case two of aerodynamics. You did say aerodynamics
were negligible before so I must assume you mean this is another scenario
that could result in the same effect independently or in concert.

In addition to not buying the wing theory, I don't know if they can really
be independent. Let me draw your attention to the pantagraph video posted.
Look at the shape in the desk that was removed during the collision. It has
every indication of looking like the 'deck of cards' iron was not only not a
wing, but rather it went through edge on slicing in the wind direction just
like a weather vane. Not much lift generated by weather vanes. Itis worth
noting here that the comic book (whatever, atlas as you called it?) may have
just gotten stuck in the hole in the desk in an inelastic collision and was
simply torn through like tissue paper.

I said:
"What seems much more likely to me is that the object was hurled
horizontally in which case it goes through the window and through the
breaking glass is altered down at the desk which is plausible for an object
hurled horizontally AFTER passing the maximum height (vertex) in its
parabolic arc."

You replied:
"I would be more likely to believe this if it had just punctured the cheap
particle board desk. But the damage to the atlas is more impressive. A stack
of paper like that can absorb a lot of energy. It looks to me like the
object had considerably more velocity than I'd expect from something tossed
by hand. Maybe somebody a few blocks away built a potato cannon?"

Heh. I don't have the answer, but we both have the same problem. I do have
more suspicions now. While I previously mentioned I was very impressed by
the book damage like yourself, I have since reconsidered (see above). Let's
look at the collision and get a feel for how much force was applied for how
long. Such a collision, usuing the speed of sound in wood, not to mention
steel I estimate would last 0.1 milliseconds. That is quite an impulse with
400,000+ Newtons. The momentum of the thing, neglecting the window and
shade, was 18.9 kg-m/s at the average terminal velocity of 47 m/s (105mph).
A strong troublemaker who has access to slag could probably hurl the piece
at a measly 18.5 m/s (42mph), exactly 40% of that number, say 160,000 N over
the same instant. My conclusion is that while your logic is sound, it
doesn't fit the observations as well as a horizontal hurl. I have failed to
take into consideration rotational inertia from a thrower, too. I don't
think this changes the story as both cases can claim it. What I do see in
the numbers is that any piece of iron that lands on an edge can do great
damage by playing all that inertia against a tiny spot. There is one more
comment. There was no other damage mentioned. I suspect the nealy one
pound piece of iron (whoa, how much more lethal than stone) had a
comparatively soft (non-damaging) landing on the chair, or perhaps the
floor, after squeezing through the thin hole it made by hitting edge on.

Just to be comfortable with this, I just undid the small base of my big
tripod (metal from MMC :-)). It is 8cm x 8cm x 1cm and weighs 25% more than
the meteorthing but is much smoother and nearly double the flat area on each
side. It is a dangerous weapon. I have no doubt I could easily make a
clean break in a major bone with it with a angry throw. And a bone has much
greater strength than what the meteorthing did.

Finally, all of this speaks nothing about the sheer coincidence that the
hole in the shade, desk and up&down orientation of the thing is exactly at
right angles to the wall. I followed the lines of perspective in a still
from the video.

Again, not dening your hypothesis is possible, just much more improbable.
The one irreconcilable problem I see is the loss of height from the hole in
the window to the shade. It seems lined up with the desk but way too much
to have contunued on to the far corner of the desk, which puts all interior
angular measurements suspect in my current view.

Best wishes and Good Health,
Doug
Received on Wed 07 Mar 2007 08:03:42 PM PST