The 9/11 Forum

einsteen wrote: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

Oh yeah, forgot about that.

appeasing, i feared allready this was going into "faster than freefallspeed"

Dictator Cheney wrote:appeasing, i feared allready this was going into "faster than freefallspeed"

If that really happened then a possibility is: It collapsed internally and after it gathered enough speed it made contact with the outer shell which was pulled due to the velocity of the internal collapse and pulled down....

serious now, using smearograms I would say it is a very fast acceleration, near g, some factors that may lower the value

- overestimation of the height of the building (186m vs 174m)
- overestimation of the distance of a story not L/47 but (L-value)/47
- framerate of the video not genuine, PAL (25fps), NTSC(29.97fps), it was 30fps which is none of them, one needs the Naudet dvd.

i have the Naudet DVD

Great, it is only a small cropped part if I remember well, firemen -> tower -> it's gone man

Do you have the possibility to cut a part from the RAW mpeg2 file and put it on megaupload or rapidshare ? You probably need to have a decrypter to access the .VOB files (=mpeg2) and a mpeg2 cut tool, I could provide them if you need it.

See: http://www.911blogger.com/node/17685 .

Using physics toolkit, he finds acceleration within 1 percent of g for about 2 seconds (measuring from the roofline).

Has anybody tried to figure out what the breakdown of 'strength' is between pushdown on just the outside of the building, versus pushdown of the interior? Since nobody describes WTC 7 as having an exoskeleton, it's got to be relatively easy to collapse the exteriror.

Still, the fact that the acceleration decreases from essentially free fall means there must be more resistance. So, even if you assume the interior collapsed 'first', I don't think you can assume that it was completely disconnected from the exterior.

Well, I guess I'm not even sure of that! CD means controlled demolition. I wouldn't say that it's completely impossible that the interior was essentially disconnected from the exterior. Who's to say that the acceleration decrease is not merely due to the strength of the exterior, and that it was completely disconnected from the interior, but that the CD was designed to destroy the exterior, load bearing columns (sequentially), from the bottom, but for only the first 2 seconds.*

Intuitively, this makes sense ito preventing the building from falling over. Once all that mass gets going in a completely vertical direction, what sort of forces could initiate a pivoting?


* Anybody care to expound on how best to define "exterior, load bearing columns"? I really haven't looked at the details of the WTC7 structural elements much at all.

i dont have the tools. are they freeware? where can i find them?

There is a new thread about Bldg 7 freefall at JREF (http://forums.randi.org/showthread.php?t=123340). Inside, you'll find nonsense spoken with great authority. I'd respond over there, but I don't need the resulting fecal paintjob, so I'll do it here just to put the record straight.

rwguinn wrote:In answer to why you can't measure acceleration from the video:
No accurate time reference, (frame rate can vary) no accurate (I.e., undistorted (unless you have video from straight on, perpendicular-to-the wall-at-the-same-elevation)) distance reference. A mis-measurement of as little as .05 seconds or 1 foot is significant. The time reference is also very short. Accumulated error is huge.

and later:

rwguinn wrote:Lots of videos, access to the originals (uncompressed), from many angles--plus they had access to construction details, and a whole hell of a lot better analysts and equipment than the average troother...

Almost all of this is wrong. See below for the one exception.

>>No accurate time reference

I actually can't address this because I've no idea what he's talking about, unless (despite separation by comma) it's tied into the next parenthetical comment:

>>(frame rate can vary)

Yes, from one video to another, but so what? As to variance within one video, it's insignificant and is not cumulative. An hour video encompasses an hour of real time to within a small fraction of a second. Variance of frame rate is a sharp distribution about a mean and, in digital cameras, is essentially identical to an accurate timestamping datalogger. I've lamented the poor quality of internet videos, especially frame rate conversions, but it has nothing to do with this.

>>no accurate (I.e., undistorted (unless you have video from straight on, perpendicular-to-the wall-at-the-same-elevation)) distance reference.

I guess he's never heard of perspective correction. In some cases it's not even necessary to know the scene geometry to correct for perspective distortion, a (x,y) scaling map might be obtained from known lengths of objects at various locations in the scene. However crude, this sort of correction field can get you to within a few percent which is good enough for many purposes. He betrays his lack of understanding by claiming the ideal orthogonal shot does not need perspective correction. Oh yeah?



(By the way, the object in the image is a flat checkerboard viewed head-on) Generally, it does require correction, with the exception discussed next.

Despite the above, for judicious choice of video, it sometimes suffices to get one vertical measurement of a known distance (# of floors) and use this as a single scaling factor as has been done in this thread. Many 911 videos are shot at considerable distance with zoom, giving a small total view angle hence allowing for small angle approximations for all but the most exacting work. In other words, with the right video, distortion is not even an issue.

>>A mis-measurement of as little as .05 seconds or 1 foot is significant.

I don't disagree with this on its face but it's pretty vague. Significant in what way? Enough to prevent meaningful measurements from being taken? Hardly. As with above, such errors are NOT cumulative but represent a(n essentially Gaussian) distribution which, with enough data points, can provide reasonably sharp discrimination. Again, just like any other data recorder.

>>The time reference is also very short.

I have no idea what this means or how it pertains to the discussion.

>>Accumulated error is huge.

As I pointed out above, the accumulated error is insignificant.

>>Lots of videos

I've looked at countless videos.

>>access to the originals (uncompressed)

I can buy DVDs, too, and I have.

>>from many angles

Ibid I've looked at countless videos.

>>plus they had access to construction details

What do you know, he's right about that.

>>and a whole hell of a lot better analysts and equipment than the average troother

All that's needed is a computer and freeware. I can't speak for the 'average troother' (whatever that is), but I write my own software to extract data from frames.

There, I feel better. I hate misinformation. I've posted my own caveats concerning measurement to obtain 'actual instantaneous' acceleration, and these stand. The remarks by rwguinn, however, are worthless.

PS Another poster, technoextreme, had this comment about rwguinn's assessment:

technoextreme wrote:I completely agree with everything he said.

Normally, I wouldn't take a potshot at something like this, but considering this was the first reply in the thread:

technoextreme wrote:As an engineer I can say with a 100% certainty that you can't measure acceleration using the videos.

I was going to let this slide because technically the measurement is position, but now I have to say I'd never want that engineer in my employ. It's OK to not know, but it's not OK to pretend you do.

Sorry for the OT, Hambone and Dr. G.

As truther I can say that you can't measure acceleration with a 100% certainty using the videos.



PS: DVD's are comressed afaik.

Dictator Cheney wrote:PS: DVD's are comressed afaik.

I think you're right so I'll correct my statement to say that I am in possession of video of sufficient quality to obtain accurate measurements. NIST does not have an edge because they can get their hands on an original AVI or MOV from someone's digital still camera, or an MPEG from a video camera, these are all compressed at the source. As far as I know, the only common uncompressed format is AVI full frame and no device uses it, hence all video is compressed. A good DVD is a faithful-as-possible copy of the source.

Edit: I also need to clarify my response to the claim of varying time rate. There are slow motion videos, yes, but they are obviously so and aren't used for these purposes. Resampling to a different frame rate, common and annoying, is an unrelated issue.

I was about to post about those clowns over at JREF, but you guys beat me to it! As usual, when a JREFer is confronted by somthing he doesn't like, he says it can't be true!

Moderator Warning

2. Abusive language, profanity and insults will not be tolerated whether directed at an individual or a group.

/Administrator

Anyway, back on topic!

I think David Chandler has done some good work with his little video but I do have one complaint. This is with his selection of t(zero). Even looking at the velocity plot he displays in his video I would move his t(zero) cursor a tad to the left which would slow his acceleration significantly ......

This has prompted me to do some sensitivity tests on some of my own collapse plots. I have taken one of my "best efforts", which generates a very smooth curve, and looked at the effect of shifting t(zero) by just 0.1 seconds. When I do this I get these two curves:

The original plot fits to:

Drop = -0.077t^3 + 4.8433t^2 -0.2123 t + 0.0669

And the plot shifted by 0.1 seconds gives:

Drop = -0.0176t^3 + 4.4721t^2 - 0.5854t + 0.0034

Hence for the acceleration, by double differentiation, we have:

Original plot:

Accel = 9.6866 - 0.462t

Plot shifted by 0.1 seconds:

Accel = 8.9442 - 0.1056t

Thus we see a marked change in the calculated acceleration for just a 0.1 second shift. This is why I have argued that the measured acceleration is about 9.2 +/- 0.6 m/s^2.

This means the collapse of WTC 7 is still VERY fast, and WAY FASTER than NIST says, but certainly NOT at free fall!

what is the "exact" gravity in New York?

DC:

From "Fundamentals of Astrodynamics and Applications" by D.A. Vallado:

Because the earth is not exactly spheroidal, g varies with latitude.

g at the equator is 9.780325 m/s^2
g at the pole is 9.832185 m/s^2

For NYC you need to use:

g(NYC) = {(1 + k.sin^2(lat))/(Sqrt(1 - e^2.sin^2(lat))}

with e = the earth's eccentricity = 0.006694

and k a constant = 0.00193185

From this mess I get g(NYC) = 9.8167 m/s^2 but you may want to check my math!

Anyway, I always use g = 9.81 m/s^2 which I think is plenty good enough for the present debate!

Dr. G:

Thank you

and i fully agree that 9.81 is good enough, just wondered

Dictator Cheney,

http://rapidshare.com/files/144472564/M ... r.zip.html

DvdDecrypter is able to extract the files from a original dvd. The .vob files are interesting, you can easily use mpeg2cut to open a .vob file, select a part from it and save a few MBs...