The 9/11 Forum

Major Tom:

Just to clarify: the acceleration range I quoted is for ALL the data I have looked at and involves at least TWO different videos. The precision on any ONE given plot is, of course, much better.

The main problems in getting precise numbers are:

1. Defining t(zero), the moment the roofline starts to move downward.

2. Measuring each drop distance. This is most difficult during the first second of motion. Here an uncertainty of 1 meter is as large as 30 % of the drop. After that the drop error gets progressively smaller. Thus at about 4.5 seconds the building has dropped about 86 meters. The uncertainty in the drop distance measurement might now be 2 meters, but this is only about 2 % of the drop.

I plotted a simple curve in maple

plot(-x^2,x=-1..1);

Scaled the axis a little bit, and copy and pasted it onto the image from yesterday

Tada


http://i38.tinypic.com/2cwvlza.gif


Same answer


Further if we think about the Sandia test the perspective is relevant, in this case if the camera
was at infinity (instead of the base [Naudet bros]) the acceleration would be even more.

Major Tom,

Since a big mass is involved the curve will always be smooth, accelerations above g are inconsistent with physical principles, peak accelerations are rubbish due to noise.

My conclusion is that the 18 stories drop in 5.4s that NIST gives is not untrue but is formulated weakly. If it falls 18 stories in 3.7 seconds (what happened) then it also drops 18 stories in 5.4 seconds. One could also add the time the penthouses started moving, but isn’t the GLOBAL collapse the interesting part ??

It is like wtc7 turned into a liquid during the pre-weakening phase.

...that was good to the last drop.

Since a big mass is involved the curve will always be smooth, accelerations above g are inconsistent with physical principles, peak accelerations are rubbish due to noise.

Just to explain the past comment: An academic on a different forum claimed there were momentary accelerations exceeding g. He claimed this could be proof of thermobaric weapons being used. I know this is normally unphysical, but in the case of thermobarics, an overpressure wave is followed by an extreme underpressure, hence creating a "sucking down" of an object already falling. And hence, accelerations which can momentarily exceed gravity.

I can now understand that the margin of error involved in measuring momentary accelerations from x(t) are too large and I like the third order polynomial fit by Dr G.

It has to be a third order minimum to account for changes in a. Though, of course, you will never discover anything other than a constant a or a(t)=ct using this method.

Don't think it is important though unless someone gives a reason otherwise.

Major_Tom wrote:OneWhiteEye, does this margin of error seem reasonable ("doable") to you?

Yes, while I was pontificating, Dr. G done did it.

Good work, Dr. G! As always.

einsteen wrote:I plotted a simple curve in maple

plot(-x^2,x=-1..1);

Scaled the axis a little bit, and copy and pasted it onto the image from yesterday

Tada

http://i38.tinypic.com/2cwvlza.gif

Sometimes you come up with the best pictures. It speaks volumes.

My conclusion is that the 18 stories drop in 5.4s that NIST gives is not untrue but is formulated weakly. If it falls 18 stories in 3.7 seconds (what happened) then it also drops 18 stories in 5.4 seconds.

It's this sort of thing that got you your new job, isn't it? {PHP smiley insertion error - code 624} I'm not sure NIST will get it, though.

One could also add the time the penthouses started moving, but isn’t the GLOBAL collapse the interesting part ??

Agreed.

I see Dictator Cheney has emailed NIST asking about the deflection scale (and provided a nice case for it being 1:1 scaling). It's so fascinating that this is an open question at this point in time, especially since an exaggerated deflection was so confidently assumed by some.

Let me recount a familiar story about a bulge and a transit. The transit was placed, assessments were made, hours went by and things got worse. Remember, this building was creaking, groaning, leaning; it was ready to fall over! That's why a "collapse zone" was cleared, that's why the BBC can be excused for reporting the collapse 20 minutes ahead of time, as you may recall.

NIST remembers this account and that's why they include it in a bulleted series of statements which could be loosely categorized as "and here are a bunch of other reasons why we think it collapsed" but are curiously not found in their extraordinarily detailed simulation-based narrative. Noticeably, there is neither progressive bulge nor lean 3+ hours in advance of initiation - though without scaled-up deflections, maybe it wouldn't be visible.

However, with scaled deflections, the visible bulge would surely look like a rat in the belly of a snake, and the lean would appear to be the precursor to toppling. Wouldn't it?

On that basis alone, I think it's safe to say there's no scaling.

Major_Tom wrote:Just to explain the past comment: An academic on a different forum claimed there were momentary accelerations exceeding g. He claimed this could be proof of thermobaric weapons being used. I know this is normally unphysical, but in the case of thermobarics, an overpressure wave is followed by an extreme underpressure, hence creating a "sucking down" of an object already falling. And hence, accelerations which can momentarily exceed gravity.

OK, I get it. I can't speak to thermobarics. From einsteen and Dr. G's work above, I'm confident there weren't peaks exceeding g. But maybe there were thermobarics that did not suck the building down.

OneWhiteEye wrote:

Major_Tom wrote:Just to explain the past comment: An academic on a different forum claimed there were momentary accelerations exceeding g. He claimed this could be proof of thermobaric weapons being used. I know this is normally unphysical, but in the case of thermobarics, an overpressure wave is followed by an extreme underpressure, hence creating a "sucking down" of an object already falling. And hence, accelerations which can momentarily exceed gravity.

OK, I get it. I can't speak to thermobarics. From einsteen and Dr. G's work above, I'm confident there weren't peaks exceeding g. But maybe there were thermobarics that did not suck the building down.

My understanding is that thermobarics would be useless against 36,000 psi steel. They just don't generate enough pressure to damage steel. What would be the cause of a thermobaric explosion? In the CD context, what would be the purpose?

Administrator wrote:

OneWhiteEye wrote:

Major_Tom wrote:Just to explain the past comment: An academic on a different forum claimed there were momentary accelerations exceeding g. He claimed this could be proof of thermobaric weapons being used. I know this is normally unphysical, but in the case of thermobarics, an overpressure wave is followed by an extreme underpressure, hence creating a "sucking down" of an object already falling. And hence, accelerations which can momentarily exceed gravity.

OK, I get it. I can't speak to thermobarics. From einsteen and Dr. G's work above, I'm confident there weren't peaks exceeding g. But maybe there were thermobarics that did not suck the building down.

My understanding is that thermobarics would be useless against 36,000 psi steel. They just don't generate enough pressure to damage steel. What would be the cause of a thermobaric explosion? In the CD context, what would be the purpose?

Oh, I was just keeping an open mind. Or joking, if you prefer.

i am totaly confused

OWE, DrG,

Some old work

http://www.physforum.com/index.php?show ... 83&st=8370

You did the work OWE, but I'm not sure about counting stories, but they are visible

Well now we have WTC 7 smearograms a plenty! Looks like OWE has been holding out on us,...... sorry I forgot those old PhysOrg examples.

As I mentioned before, there are obviously some variations between all the avilable WTC 7 collapse plots but I think we can agree that the effective g is somewhere between 8.5 and 9.5 m/s^2, or Einsteen do you really stand by a g greater than 9.8?

Whatever! The final, "global" collapse is pretty darn FAST!!!!

One thing I would add is that there is no detectable induction phase to the collapse. It starts like someone cutting a rope or turning off an electromagnet.

Of course not more than 9.80665 m/s^2, I'm no dr. only a simple M.Sc. but 5 years physics should be enough to know that.. (also that the earth wouldn't be swallowed by a black hole this morning) but the global collapse was even more rapid than I thought, it would be nice to compare the collapse energy per story based on the total collapse time (that must be hard to measure) with the value based on the initial acceleration. Although the mass is unknown the things could be compared using the ratio E1/M, are you going to submit your values also to the nist ?

Einsteen:

Oui!