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Author Topic: What is Free Fall?  (Read 62782 times)

Offline Aemilius

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Re: What is free fall?
« Reply #100 on: 28/12/2013 09:34:24 »
So.... Was I mistaken?

....I've gone to a lot of trouble to explain how torsional stresses are incurred in a collapsing steel-and-concrete structure.

And I really just want to say, once again, how much I appreciate your patience and the time you've taken to discuss it with me.... I'm very glad to have met you.

There are, however, other features of the aerostatic model that I see as being possibly/probably problematic though, aside from the obvious fact that the actual conditions required for free fall are never created in the course of this model playing out....

The "piston" effect....

The West Penthouse breaks in half immediately upon beginning its descent. I believe this would have largely negated the piston effect that must be relied on to explain a catastrophic build up of aerostatic pressure since the "piston", right from the start, falls in at least two pieces, so there wouldn't have been any kind of a "seal" there between the "piston" and the "cylinder wall" to fascilitate any aerostatic build up of pressure, and in addition to that, further on, there would have been nothing to prevent any aerostatic build up that did occur from simply venting in an upward direction out through the gaping hole left in the roof of the building after/as the volume of rubble that had made up the West Penthouse continued its descent within the building.
 
The "bursting"....

After reviewing the video multiple times, the evidence for an "aerostatic blowout" or "bursting" event that you point to in the video emerging from the facade at the far end of the building about five stories beneath the West Penthouse, amazingly, actually appears at almost the exact same time as the beginning of the descent of the West Penthouse itself. I believe this could explain windows being blown out on one or maybe two floors immediately below the penthouse, but it doesn't seem at all probable at that point, after just the first few feet of its descent, that the West Penthouse could possibly have developed enough aerostatic pressure within the building to have blown out even one window five stories below, let alone the cladding, after having only fallen such a short distance prior to the appearance of the purported "bursting" five stories below.

The "shockwave"....

The evidence for the "shockwave" that's seen texturally rippling accross the buildings facade, as with the "bursting" event, amazingly, also appears at almost the exact same time as the beginning of the descent of the West Penthouse itself, and again, just as with the "bursting" but even more so, it doesn't seem at all probable that, at that point in its descent, it could possibly have developed anywhere near the level of aerostatic pressure within the building that would be required to generate such powerful and wide spread effects, racing throughout the building and buckling columns as it reverberated throughout the 610 foot tall structure that covered an area greater than that of a football field. Although the windows are seen to shatter in a way that's suggestive of a "shockwave", inexplicably, there is no visible indication of any accompanying ejecta similar to the "bursting" event, no papers, no smoke, no dust.... nothing. The windows merely shatter which, to me, is really much more suggestive of a slight structural flexure having occurred as a result of the first violent initial moments of descent of the West Penthouse as it tore itself away from other structural components, thereby distorting the normal dimensions of the window frames and causing the windows to shatter progressively in rapid succession across the facade of the building, a process that would, like the "shockwave", also have continued for the entirety of the buildings descent, or until all the windows shattered.

Thanks again for your time and all your responses Mr. Calverd.
« Last Edit: 28/12/2013 20:31:56 by Aemilius »
 

Offline Aemilius

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Re: What is free fall?
« Reply #101 on: 28/12/2013 20:55:20 »
Just a note to add that those are all just subjective debatable observations. The main objection that thoroughly deflates the aerostatic model is still that it badly fails to display a close enough behavioural correspondence to the confirmed observation of gravitational acceleration, since there is no point at which the conditions required for it to occur actually arise in the model as it plays out to completion. We could revisit the rest if/when that hurdle is behind us.
« Last Edit: 28/12/2013 21:21:11 by Aemilius »
 

Online alancalverd

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Re: What is free fall?
« Reply #102 on: 29/12/2013 01:05:55 »
Remember that you don't need spectacular ejecta to weaken the structure. A few extra pounds internal pressure per square foot of cladding is all that is required to destroy the lateral and torsional stiffness of the building. And in a tall modern building the windows are a significant contributor to the stiffness of the structure. 

More importantly, the collapse of the west penthouse isn't the cause of the failure but an early visible effect. You stated that the primary cause was a fire some way below, which presumably weakened some of the steel uprights. Once the steel starts to buckle at one point, the weight of all the floors above it starts to move downwards and between them they create quite a substantial piston. One foot of collapse will increase the air pressure by about 1 lb/sq ft in the storey immediately below. The problem is that you can't see the internal floors moving. 

And finally, aerostatic blowout doesn't cause collapse by taking away all the supporting structure. All it does is reduce the stiffness of the building, which then accelerates as seen.

Quite clearly the conditions do arise for near-g acceleration, and there is no evidence for a magical dematerialisation of the internal structure, so if you don't like the obvious mechanism I proposed, you will have to propose a better one! What do you suggest?
 

Offline Aemilius

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Re: What is free fall?
« Reply #103 on: 29/12/2013 02:01:00 »
Thank you Mr. Calverd, let me ruminate on all that for a bit. Before I forget though, just for the sake of clarity....

Was I mistaken or not (reply 99) as to what you originally constructed your theory/model around? 
« Last Edit: 29/12/2013 05:47:46 by Aemilius »
 

Online alancalverd

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Re: What is free fall?
« Reply #104 on: 29/12/2013 16:30:12 »
The key is in an earlier exchange, around 67 - 69, where I discussed the rigidity of a compound structure . A simple brick shed derives all its properties from the compressive strength of the brick shell, because there isn't anything else, so if you blow the bottom outwards it will collapse as a simple piston, but we are dealing here with multiple internal compartments and substantial intermediate loads in the form of concrete floors. The whole thing only works as a whole, so removing or weakening the exterior panels allows the stuff inside to collapse, particularly if the fire has been burning for long enough to weaken several interior uprights. The piston/blowout mechanism is the last straw that triggers the rapid phase of collapse, not the entire mechanism.
 

Offline Aemilius

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Re: What is free fall?
« Reply #105 on: 31/12/2013 20:05:08 »
So to review your theory....

Column 79 buckles due to heat.

Five other columns that, along with column 79, had supported the West Penthouse then buckle in rapid succession.

The now unsupported West Penthouse, together with at least the five or so floors immediately below it, begins to descend as a substantial "piston" causing a catastrophic build up of aerostatic pressure within the building below (even though it immediately breaks in half leaving a gaping hole in the roof which appears in the video to actually be venting pressure upward through the top of the roof)....



At the same time as the West Penthouse is beginning to descend, it causes the roof to move, setting in motion a process that "progressively disintegrated" the support structure below (spontaneous catastrophic progressive structural disintegration.... a novel mode of structural failure?)....



Over the next four seconds or so the "piston", as it continues to descend, gives rise to multiple aerostatic blowouts (even though no multiple aerostatic blowouts are observed that indicate anything like that occurred) that further weaken the building by blowing out cladding and windows (even though no cladding or ejecta, spectacular or otherwise, are observed being blown out anywhere)....



During the same four or so seconds, as the penthouse descends, multiple structurally transmitted stresses (generated by multiple aerostatic blowouts there's no evidence of) invisibly reverberating throughout the steel structure at the speed of sound quickly causes more columns to begin buckling elsewhere within the building....



Together, this constellation of contributing factors consisting of a progressive sequence of progressive failures occurring at various times and in different ways throughout the building ultimately ended up coming together to act in concert at some point, miraculously causing the 610 foot tall building to literally fall (right) for over 100 feet at gravitational acceleration straight down through itself as if in air (left) through the path of greatest resistance....

 
To more easily explain the acceleration of the descending upper part of the building to near-free-fall (it was, after all, only essentially free fall, not free fall, and there was always some negligible resistance that must be taken into account), we need only reverse what we have known for centuries about what actually happens during a real natural progressive structural failure so that resistance to the descending upper part of the building gradually decreases rather than gradually increasing as it actually would (increasing resistance invariably causes the descending part of any naturally collapsing structure, aside from bridges and other structures that pass through the air, to either just come to a halt, or come to a halt and then topple)....

There's no need for even cursory scenarios, analogies or any diagrams that would even begin to show how any of this could possibly happen either, because this is simply how loaded minimally stiff structures behave under these conditions (even though no similar incidents of any kind are cited or referred to that would in any way suggest anything of the sort).

It's clear that the conditions for near free fall arose (all the while remaining perfectly unclear as to exactly how the conditions required for even near-free-fall could possibly arise), and since there's no evidence of any magical dematerialization of supporting structure that rematerialized on the other side of the moon, if I don't like the obvious mechanism your proposing (which couldn't be any less obvious) I'll just have to come up with something better.
   
As far as coming up with something better goes Mr. Calverd.... you already hit a home run man! And it didn't involve any magical dematerialization of supporting structure, nor did it require any machinations like those that have, of necessity, blossomed from the aerostatic model as it's evolved, attempting to explain something it can't possibly explain. The (when applied to the building) inherently miraculous aerostatic example of the "brick shed aerostatic blowout" absolutely fails to display even a tenuous behavioural correspondence with observations. Of the two examples you originally mentioned (gas explosion and aerostatic bursting), it's actually the extraordinarily simple gas explosion example of the brick shed you mentioned that superiorly displays an immediate and solid one to one behavioural correspondence with observations and accounts for the emergence at some point of the conditions required for free fall to arise. No miracle there.... good job!

When comparing the aerostatic model to the gas explosion model, unlike the Rube Goldberg aerostatic model, the gas explosion model easily replaces all of the above with a single sentence....

Judging by the confirmed observation of a significant period of gravitational acceleration, a high probabilty exists that a gas or other type of explosion or event must have occurred that was powerful enough to quickly remove the support from beneath the upper part of the building, either all at once or incrementally in advance of its descent (right), permitting it to descend at gravitational acceleration for the observed period of time and under the conditions required (left) for free fall to occur....


I think I'm done with the aerostatic model, but there is one thing I'm really curious about though. When you originally considered/came up with those two examples.... How did you manage the Olympically executed Herculean pole vault over Occams razor?

Can you try to delineate for me in a simple step-by-step way what the line of reasoning was that led you to choose the aerosatic model over the gas explosion model as being most probable (if you recall)?
 
« Last Edit: 06/12/2015 05:00:23 by Aemilius »
 

Online alancalverd

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Re: What is free fall?
« Reply #106 on: 31/12/2013 20:23:37 »
No.

You said the fire was some way down the building (8th floor?) so the movement of the west penthouse clearly isn't the first stage of collapse - it just happens to be the first externally visible sign. We have no idea of the state of the intermediate floors.

Had there been a gas explosion you would expect to see (a) the shockwave emanating from the level of the explosion, not from  near the top of the building (b) significant ejecta from that level and (c) not a lot of collapse since the steel frame would probably have withstood a contained burst at one level - more likely to have blown out the windows symmetrically without damaging the steelwork or breaching the floors.

Intentional explosive demolition is done by attaching simultaneous high-explosive charges (gas is not a high explosive) to the steelwork itself, to produce the kind of torsional failure that I think probably occured here. 

And remember that aerostatic blowout isn't the whole picture - it merely renders the structure "unstiff" and therefore unable to support torsional loads as the floors collapse.

I like the idea of polevaulting over Occam's razor. But given the starting conditions you offered, with no suggestion or evidence of a gas explosion, much less of a continuing fire during the descent, I wouldn't have used gas as the pole.

Quote
a process that "progressively disintegrated" the support structure below (spontaneous catastrophic progressive structural disintegration.... a novel mode of structural failure?).

That's what you can see on the video - at least the external evidence. Our task is to explain it.
« Last Edit: 31/12/2013 20:26:49 by alancalverd »
 

Offline Aemilius

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Re: What is free fall?
« Reply #107 on: 31/12/2013 21:03:58 »
Thank you for that in depth analysis of why a gas explosion wouldn't bring down the building, but I didn't limit the cause of the explosion to gas alone, what I said was....


"....a high probabilty exists that a gas or other type of explosion or event must have occurred that was powerful enough to quickly remove the support from beneath the upper part of the building, either all at once or incrementally in advance of its descent...."
« Last Edit: 24/01/2014 14:20:45 by Aemilius »
 

Online alancalverd

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Re: What is free fall?
« Reply #108 on: 01/01/2014 11:38:19 »
As we discussed earlier, if you remove the support explosively, the upper stories will accelerate immediately at a rate close to g. They didn't.
 

Offline Aemilius

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Re: What is free fall?
« Reply #109 on: 04/01/2014 18:15:22 »
Intentional explosive demolition is done by attaching simultaneous high-explosive charges (gas is not a high explosive) to the steelwork itself, to produce the kind of torsional failure that I think probably occured here.

No. Intentional explosive demolition is rarely done by attaching simultaneous high-explosive charges to the steelwork.... it's done by attaching precision timed high-explosive charges to the steelwork.
 
Quote
a process that "progressively disintegrated" the support structure below (spontaneous catastrophic progressive structural disintegration.... a novel mode of structural failure?).

That's what you can see on the video - at least the external evidence. Our task is to explain it.

Misplaced concreteness, Mr. Calverd?

As we discussed earlier, if you remove the support explosively, the upper stories will accelerate immediately at a rate close to g. They didn't.

And as I just pointed out, simultaneous detonation of the charges is rare. In fact, since the detonations can be (and routinely are) precisely timed, the door is thrown wide open for a variety of possible outcomes. One could quickly detonate all the charges at once for immediate acceleration to free fall, or one could detonate them slowly over time, including a period of free fall at any point one wished.... so that's not ruled out yet. 

To sum up....

My position is that not only is it impossible for the lower part of the building to have progressively/naturally collapsed in any way that could have resulted in the upper part of the building descending at anything near gravitational acceleration (below left) for any period of time during it's descent (below right), but also that there is absolutely no mode or combination of modes of progressive/natural structural failure driven solely by gravity that could ever give rise to the conditions required for anything near free fall to occur at any point during its descent....


Your position (unless I'm misunderstanding things incorrectly) is that not only is it somehow possible for the lower part of the building to have progressively/naturally collapsed in a way that would result in the upper part of the building accelerating as it descended (below right), but also that, driven solely by gravity, it would actually continue to accelerate so nearly to gravitational acceleration (below left) as to require careful calculation for any difference between the two to be detected....


I don't see how it can to do us any good to continue researching all the possible structural failure modes (or even make up new ones) in the hope of finding that special one that can create the conditions required for even near-free-fall to occur because it's physically impossible.

When it comes to your "catastrophic aerostatic blowout"....


...."speed of sound stress propagation shockwaves"....


...."spontaneous progressive structural disintegration" theory....


We'll just have to shake hands, agree to disagree.... and return to our respective universes.

P.S. - Diogenes, your posthumous guest and loyal eulogist, told me to tell you that.... "Though Perpetual Motion may be impossible, take heart, for the 'Oxford Bell' continues to ring!"
« Last Edit: 19/02/2016 08:58:40 by Aemilius »
 

Online alancalverd

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Re: What is free fall?
« Reply #110 on: 04/01/2014 23:50:41 »
'
Quote
I don't see how it can to do us any good to continue researching all the possible structural failure modes (or even make up new ones) in the hope of finding that special one that can create the conditions required for even near-free-fall to occur because it's physically impossible.

Suit yourself, but on my planet we assume that what we see and measure is indeed physically possible.

Never believe a philosopher - especially one that lives in a barrel. Trust your own eyes.
 

Offline Aemilius

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Re: What is free fall?
« Reply #111 on: 05/01/2014 11:38:39 »
....on my planet we assume that what we see and measure is indeed physically possible.

Apparently you can see and measure things that aren't even there.... Must be some planet!

At almost the exact same time as the West Penthouse begins to break up and descend, there's evidence of venting five or so stories below it and out the top of the building above it as well.... but you see a massive multi-floor piston instantly causing a catastophic aerostatic blowout and powerful shockwave (all created by just the first few feet of the descent of the West Penthouse).
 
There's really nothing but a puff of smoke/dust at the first purported blowout.... but you see evidence of not just one but multiple aerostatic blowouts and shockwaves having occurred.

At almost the exact same time as the West Penthouse begins to descend, windows begin to shatter down and across the facade over multiple floors with no accompanying ejecta whatsoever, no smoke, no papers, no dust, nothing. Only a structural flexure can explain that (there was no pressure behind the windows).... but you see a powerful aerostatic shockwave racing accross the building through both walls and floors (all created by just the first few feet of the descent of the West Penthouse).

Though not one panel of cladding is seen to be dislodged during any of this.... you see evidence of multiple aerostatic blowouts having dislodged enough cladding/windows around the building to seriously affect its rigidity.

Though the entire facade of the building remains largely intact as it descends and there's no visible sign of any buckling or structural failure having been caused by the roof moving.... you actually see a mysterious new failure mode that can cause spontaneous widespread catastrophic progressive disintegration of steel columns.
 
Is there even one recorded incident of anything like this ever actually happenning that would tend to support your assertion about how the building came down?
« Last Edit: 05/01/2014 13:10:08 by Aemilius »
 

Offline Aemilius

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Re: What is free fall?
« Reply #112 on: 05/01/2014 13:28:50 »
Interesting to note that I'm not getting slapped around (as I'm accustomed to), but neither is there any chorus of support for your explanation either Mr. Calverd. Even if one subtracts my 1,749 edits, there're still quite a few "views of the thread.... Where do the rest of you fall on this issue?
« Last Edit: 19/02/2016 09:06:50 by Aemilius »
 

Offline Bored chemist

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Re: What is free fall?
« Reply #113 on: 05/01/2014 14:10:04 »
I can't speak for anyone else, but I didn't bother to get involved in this discussion because I got bored of refuting absurd claims about the tower falling years ago.

All the evidence supports Alan's point of view.

It fell down because some shits flew a plane full of jet fuel into it.
That fire heated the steelwork until it failed. The upper floors fell down and, since skyscrapers are not designed to take massive vertical shock loads, the rest collapsed.
It's not possible to analyse the video footage accurately enough to measure the acceleration to a high enough precision to rule out near free fall.
Typical video pictures are about 500 hundred lines high, and the image falls through about half the frame height so, at best you can measure the height of the building in each frame to about 1 part in 250.
That's simply not enough precision to rule out the suggestion that the building fell down.

You have, essentially, no evidence; but you have wasted 5 pages talking about it.

Anyone joining in with the discussion to point this out to you wouldn't have stopped you rambling on about it.
 

Offline Aemilius

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Re: What is free fall?
« Reply #114 on: 05/01/2014 15:07:43 »
I can't speak for anyone else, but I didn't bother to get involved in this discussion because I got bored of refuting absurd claims about the tower falling years ago.

Well, like I said, I'm a relative newcomer to the internet so, sorry if it's already been exhaustively discussed. As far as absurd claims go, I haven't made any claims. So far we've just discussed Mr. Calverd's absurd claims.

All the evidence supports Alan's point of view.

And precisely what "evidence" is that Bored chemist?

It fell down because some shits flew a plane full of jet fuel into it.[

Indicating that you haven't a clue as to what Mr. Calverd and I have been discussing for the last 100 or more posts!

That fire heated the steelwork until it failed. The upper floors fell down and, since skyscrapers are not designed to take massive vertical shock loads, the rest collapsed.

Conclusively proving that you haven't a clue as to what Mr. Calverd and I have been discussing for the last 100 or more posts!

It's not possible to analyse the video footage accurately enough to measure the acceleration to a high enough precision to rule out near free fall.

Hah! Yeah, sure pal, everyone knows your assessment trumps the NIST.... Ridiculous! High enough precision to rule out near free fall?  What does that even mean?
 
Typical video pictures are about 500 hundred lines high, and the image falls through about half the frame height so, at best you can measure the height of the building in each frame to about 1 part in 250.

I'll get you the email address for the NIST. Why bother me with it? Take it up with them!

That's simply not enough precision to rule out the suggestion that the building fell down.

Not precise enough to rule out the suggestion that the building fell down? Hilarious! You're a riot man.... you're going to be bigger than Jerry Seinfeld! 

You have, essentially, no evidence; but you have wasted 5 pages talking about it.

I don't know about the five wasted pages bit, but somehow you've managed to squeeze a whole truckload of waste into just one post. Very impressive!  And when the hell did I ever say I had any "evidence" of anything? Got a quote.... Or would you like to just keep making it up as you go along?

Anyone joining in with the discussion to point this out to you wouldn't have stopped you rambling on about it.

Well, Bored chemist, I can say with confidence that your joining the discussion certainly didn't change a thing! Ask yourself this Einstein....

Why would a bunch of jibberish coming from someone who doesn't even know what the topic is change anything? Your whole post is a bunch of worthless crap!

If you're supposed to be the chorus of support for Mr. Calverd.... woe to Mr. Calverd.

« Last Edit: 19/02/2016 09:12:59 by Aemilius »
 

Online alancalverd

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Re: What is free fall?
« Reply #115 on: 05/01/2014 15:29:52 »
Quote
Though the entire facade of the building remains largely intact

A couple of inches is all it takes.

Anyway, enough of my hypotheses. What do you think happened?
 

Offline Aemilius

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Re: What is free fall?
« Reply #116 on: 05/01/2014 15:34:05 »
Right, now it's a couple of inches, before it was a couple of feet.... C'mon!

What do I think happened? I'm nobody.... Who cares what I think? You're the Physicist, you're the one who's supposed to deliver scientific truth to me, not the other way around. I don't feel you're doing that (nothing personal).
« Last Edit: 28/01/2014 16:21:27 by Aemilius »
 

Online alancalverd

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Re: What is free fall?
« Reply #117 on: 05/01/2014 15:48:11 »
If we are being pernickety, it's Dr Calverd, if you don't mind.
 

Offline Aemilius

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Re: What is free fall?
« Reply #118 on: 05/01/2014 15:49:58 »
You mean persnickety? Well we're not so it will remain Mr. Calverd, unless you really find it objectionable.... Would you prefer "Dr. Calverd" Mr. Calverd? From now on I'll refer to you as "Dr. Allen Caverd Ph.D. Physicist"
« Last Edit: 06/01/2014 00:12:17 by Aemilius »
 

Offline Aemilius

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Re: What is free fall?
« Reply #119 on: 05/01/2014 17:05:46 »
Now that we've addressed all the etiquette issues Dr. Allen Calverd Ph.D. Physicist.... Was there any forthcoming response to reply 111?
« Last Edit: 08/01/2014 02:27:11 by Aemilius »
 

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Re: What is free fall?
« Reply #120 on: 05/01/2014 17:12:09 »
Alan
 

Offline Aemilius

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Re: What is free fall?
« Reply #121 on: 05/01/2014 17:18:42 »
Right (terribly sorry), "Dr. Alan Calverd Ph.D Physicist".... got it. Was there an actual response to reply 111?
« Last Edit: 06/01/2014 00:09:59 by Aemilius »
 

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Re: What is free fall?
« Reply #122 on: 05/01/2014 17:26:29 »
#115, IIRC
 

Offline Aemilius

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Re: What is free fall?
« Reply #123 on: 05/01/2014 17:53:46 »
So all of reply 111....

Apparently you can see and measure things that aren't even there.... Must be some planet!

At almost the exact same time as the West Penthouse begins to break up and descend, there's evidence of venting five or so stories below it and out the top of the building above it as well.... but you see a massive multi-floor piston instantly causing a catastophic aerostatic blowout and powerful shockwave (all created by just the first few feet of the descent of the West Penthouse).
 
There's really nothing but a puff of smoke/dust at the first purported blowout.... but you see evidence of not just one but multiple aerostatic blowouts and shockwaves having occurred.

At almost the exact same time as the West Penthouse begins to descend, windows begin to shatter down and across the facade over multiple floors with no accompanying ejecta whatsoever, no smoke, no papers, no dust, nothing. Only a structural flexure can explain that (there was no pressure behind the windows).... but you see a powerful aerostatic shockwave racing accross the building through both walls and floors (all created by just the first few feet of the descent of the West Penthouse).

Though not one panel of cladding is seen to be dislodged during any of this.... you see evidence of multiple aerostatic blowouts having dislodged enough cladding/windows around the building to seriously affect its rigidity.

Though the entire facade of the building remains largely intact as it descends and there's no visible sign of any buckling or structural failure having been caused by the roof moving.... you actually see a mysterious new failure mode that can cause spontaneous widespread catastrophic progressive disintegration of steel columns.
 
Is there even one recorded incident of anything like this ever actually happenning that would tend to support your assertion about how the building came down?

....is explained by reply 115....

A couple of inches is all it takes.

Oh man.... that's rich!
« Last Edit: 05/01/2014 17:56:41 by Aemilius »
 

Offline Aemilius

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Re: What is free fall?
« Reply #124 on: 06/01/2014 01:07:05 »
I think it's been an interesting and informative conversation. I see your theory Dr. Calverd, and even though I believe I've shown it to be completely baseless and thoroughly disagree with it, it's your theory and you're welcome to it. I think I'll leave it at that and just read the opinions/theories of others (if any) and respond to those, as we've reached something of an impasse.

Thanks again sincerely for all your responses Dr. Calverd, looking forward to corresponding with you again.... maybe next time something a little less controversial. Thanks to everyone else that commented too (except you Bored chemist!). 
« Last Edit: 19/02/2016 09:19:24 by Aemilius »
 

The Naked Scientists Forum

Re: What is free fall?
« Reply #124 on: 06/01/2014 01:07:05 »

 

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