# Falling objects can be faster than free fall

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#### Mendel

##### Senior Member.
In the 9/11 discussions, "free fall acceleration" is often treated as a kind of speed limit for gravity-driven falling objects. But that's a misconception. Watch this short video:

Because the ladder on the left impacts a table, it is no longer in free fall, and therefore mechanical effects can then cause acceleration or deceleration.

Nothing about the WTC collapses was free fall (excepting the parts ejected to the side that fell through clear air, which came close). Therefore, free fall acceleration is neither a necessity nor a limit.

A bit of a side question, but I have several times asked 9/11 Truth followers WHY they think it is significant that "the towers fell at free fall speed." What do they think this proves? Never got an answer -- presumably the folks I've talked to just saw the claim made, and that THIS MEANS SOMETHING, and didn't go any further into it.

Whether or not they actually fell at "free fall speed," does anybody know what CTers on 9/11 thank it would prove if they did in fact fall like that?

Great experiment!

Understanding it is made a bit complicated by gravity constantly accelerating everything that isn't resting on the ground yet, the table "swallowing" some proportion of the momentum and the kinetic energy, and the different ways in which the strings interact with the rungs (pull but don't push)

A first simple experiment that you can do at home to get a first feel for what's happening here is this:

• Find a straight bar, rod, whatever. Something solid and elastic enough. Should be of uniform properties (thickness, surface friction...) along the whole length. A ruler may do, an old-fashioned pencil, whatever.
• Place it on a flat surface with as low a friction coefficient as you can find. A polished table, laminated floor, or have it float on water.
• Note where the left end of the bar is (hold a finer next to it), and observe the motion of that end.
• Flick a finger of your right hand against the right end of the bar, perpendicular to its length.
You should observe that
1. The bar goes into rotation
2. It's center of gravity moves away from you
3. The right end moves away from you faster
4. BUT the left end moves TOWARDS you a bit, before it rotates away.
Ha, I just did a quick video:
In addition to the bar (a pencil; yes, there's a rubber on one end, making that end a bit heavier, but that makes no qualitiative difference, I could have turned the pencil around to the same effect) I placed a bottle cap on both side of the loose end. Onle the one at the bottom shoots away, demonstrating that the upwards tap on the right does work and transfers momentum downwards on the left.

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Step 2 in understanding what's happening here is to visualize before your inner eye a situation where my finger is stationary, and the pencil is moving at constant speed (sliding with negligible resiatance) till one end hits my finger.
Do I need to demonstrate that this merely represents a change of coorinate system in which I am observing the whole thing, and that all coordinate systems that merely translate (and not rotate) relative to one another are equivalent? That the same will happen - the lower bottle cap will be shot in the direction the bar is moving?

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Step 3 is now picturing the same situation, only this time, the pencil is falling till its end hits my finger . or the edge of a table, and the caps are initially falling with the pencil - AND the camera is falling (moving in unison) with the pencil, such that the pencil appears stationary in the video. We will observe the same then as before: Pencil as a whole slows down, one side (the one that hits obstacle) shoots "up" relative to camera, the other, free end however accelerates down and bats the cap down. The other cap remains untouched and keeps accelerating at g, with the camera, and thus appears motionless relative to the camera.
This shows that indeed the loose end has accelerated at >g during the collision, and the kicked cap will forever be faster than the undisturbed one.

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Now, instead of placing a bottle cap under the pencil's end, tie that end via a tight string to a rung above it that is falling in unison:
The moment the pencil hits the table and its loose end "kicks" downward, it tugs at the string exactly as it previously "kicked" the cap - and has significant energy and momentum to do work on the run above - accelerate it downward in addition to it falling already at g, thereby making it accelerate at >g.

What happens to the rest of the next rung as it gets tugged by the string? Well, same thing: The entire rung will accelerate downward, but also go into rotation, with its far end now accelerating UPwards.
This makes the next string to the next rung go limp - and the ladder and its motion a bit messy. And we should stop there.

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Now, how is this relevant to 9/11 and the WTC collapses?

Here is my usual take. Consider:

a) The core of WTC77 started collapsing while the perimeter, or at least the visible North wall, was still stationary.
b) We may assume that the core buckled several floors above ground, such that it would have fallen, after some transition, essetially at freefall, if not for the floor beams connecting it with the perimeter.
c) THEN, a fraction of a second later, the North wall buckles, generally at about the 8th floor, and after a short transistion goes, essentially, into freefall, if not for the floor beams connecting it with the core.
d) For a short moment then, both core and perimeter may be in freefall (and also there'll be some rotation; probably such that the core moves faster than the perimeter, for it started to fall earlier.
e) THEN, the core columns, with many floor beams still attached, run into a massive obstacle: The solid ground. And thus the FLOOR BEAMS experience a one-sided deceleration, on their core ends.
f) And here, we can apply what we learned in above experiments: The UPward acceleration of the core-side of the floor beams causes a DOWNward acceleration of their perimeter ends in addition to freefall, and that impulse is, IMO, what makes the perimeter accelerate, briefly, at >g!

I'd be glad if someone could draw graphics here and there. That's not my forte.

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1.) I recommend playing my short 3 s video at 0.25 speed
2.) In case you wonder what "0.0% Herb" drink this is: It's beer, alcohol-free. "Herb" does not mean "herbs" as in "herbal tea", i.e. made with spicy leaves. The german adjective "herb" means a taste that is "dry" or "bitter" and refers here to a beer rich in hops. Bitburger, the green 0.0%, if you must know

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does anybody know what CTers on 9/11 thank it would prove if they did in fact fall like that?
Article:
Basic principles of engineering (for example, the conservation of momentum principle) would dictate that the undamaged steel structure below the collapse zone would, at the very least, resist and slow the downward movement of the stories above….

Article:
[F]ree fall is not consistent with any natural scenario involving weakening, buckling, or crushing because in any such a scenario there would be large forces of interaction with the under- lying structure that would have slowed the fall.... Natural collapse resulting in free fall is simply not plausible....”

In short, incredulity, paired with the mistaken assumption that free-fall acceleration implies free fall—the ladder video shows it does not.

Since my video above evokes the same feeling of incredulity, I hope it works as antidote.

Whether or not they actually fell at "free fall speed," does anybody know what CTers on 9/11 thank it would prove if they did in fact fall like that?
It is one of a cluster of false "memes" - in the original meaning of that word. i.e. small bits off false factual assumptions that tend to replicate and spread in human communication interactions.

They include:
"Free fall proves CD", "Fall in footprint proves CD", "Falling symmetrically proves CD".

All three are false conclusions because the alleged factor is not distinguishing of CD - they also occur in what is often mislabelled as "natural collapses" itself a not precise term.

Those three are also wrong for other reasons. The first "Free fall proves CD" for a couple of reasons.

First the is so simple it is often overlooked by both sides of the argument. The motions of collapse are the results of the mechanism of collapse. NOT what started or triggered the collapse. Put simply "the starters gun does not win the race".

Second the confusion, whether through ignorance or deliberate mendacity, between "Free Fa'll" and "Free Fall Acceleration". "Free Fall" ["FF"] is a state of a body without support free to fall under gravity. "Free Fall Acceleration" ["FFA"] is the motion that results when a body is in the state of Free Fall. Those pushing the false claim for CD are usually seeking to prove, directly or indirectly that the falling body had no support - therefore CD had removed the support. That also is not a distinguishing fact. Because whilst the state of FF will cause FFA, the reverse is not true. FFA does not need zero support - it can result from net zero applied external forces.

There is more detail but bottom line - it is one of several false premises relied on by Truthers the falseness due to the assumption that the factor is distinguishing therefore proof when it is not, in reality, exclusive to the attribute it is claimed to support.

FFA does not need zero support - it can result from net zero applied external forces.
(with a considerable margin of error since the "measurements" are off grainy video)

f) And here, we can apply what we learned in above experiments: The UPward acceleration of the core-side of the floor beams causes a DOWNward acceleration of their perimeter ends in addition to freefall, and that impulse is, IMO, what makes the perimeter accelerate, briefly, at >g!

I'd be glad if someone could draw graphics here and there. That's not my forte.
I'm possibly the worst graphic artist. But I did a very crude drawing whilst I was assisting Chris Mohr in his online debate with R Gage's AE911 ghostwriters. It was very crude but AFAIK the first attempt to explain internal dynamic pull down as a potential to cause brief >'G'.

It is posted on JREF/ISF but I cannot access it - I changed my email some months back, notified ISF, and that resulted in my account becoming de-activated on ISF. I've had zero response to multiple attempts at communication to resolve the issue and have given up trying.

FFA does not need zero support - it can result from net zero applied external forces.
(with a considerable margin of error since the "measurements" are off grainy video)
I'm not sure of the relevance. My assertion of physics is true independent of any of the contentious issues about the interpretation of video evidence.

I'm not sure of the relevance. My assertion of physics is true independent of any of the contentious issues about the interpretation of video evidence.
Your "net zero" doesn't have to be an exact zero, only an approximate zero, to fit the evidence.

Your "net zero" doesn't have to be an exact zero, only an approximate zero, to fit the evidence.
Got it thank you. Yes. Near enough to net zero for any observed accuracy of the measurement. My original intention was qualitative proof of the principle NOT quantitative proof of any specific measured example.

I recommend against using this video in debunking efforts. It demonstrates the collapse of a structure with no vertical resistance. That's much more like a building that has been demolished than one that is collapsing under its own weight.

A bit of a side question, but I have several times asked 9/11 Truth followers WHY they think it is significant that "the towers fell at free fall speed." What do they think this proves? Never got an answer -- presumably the folks I've talked to just saw the claim made, and that THIS MEANS SOMETHING, and didn't go any further into it.

Whether or not they actually fell at "free fall speed," does anybody know what CTers on 9/11 thank it would prove if they did in fact fall like that?
Surely the underlying idea is that 'free fall speed' would be incompatible with any simple theory of sequential collapse, which 9/11 conspiracy theorists assume (rightly or wrongly) is the 'official' explanation of the collapse of the Twin Towers (and possibly WTC7, but that gets more complicated). Suppose, say, that Floor 100 and all its contents collapse and fall on top of Floor 99. If the impact force and/or weight of the falling debris is sufficient, the supports of Floor 99 will break and Floor 99 and its contents will also start falling. In the simplest version, the falling debris of Floor 100 will 'gather up' the material of Floor 99, and the combined debris of Floors 100 and 99 will fall together at the same speed until they hit Floor 98, and so on and on, until the debris of the entire building hits the ground. But by basic physics this is incompatible with free fall velocity after the initial collapse of Floor 100. Energy is needed to break the supports of Floor 99, and all subsequent Floors, and also to communicate a common speed to the combined debris at each stage of he process. Since the only source of this energy (it is assumed) is the kinetic energy of the falling debris, it must be 'paid for' by a reduction in the speed of the falling debris. The reduction may be slight, especially in the later stages of the process when both the velocity and the mass of the falling debris are much higher than at the beginning, but it cannot be zero. Therefore either the laws of physics or the simple theory of sequential collapse have to be given up.

I'm no expert on 9/11, so I don't know exactly what the 'official' theory is, but I assume this fairly obvious problem is considered somewhere. So far as I know, it is by and large a theory of sequential collapse, but it does allow for some refinements and qualifications, e.g. to allow for the rotation of large connected pieces of the buildings, different details of collapse for the floors, the core, and the exterior 'walls', etc. But I guess (and I'm sure someone will correct me if I'm wrong) the main answer is that the whole building doesn't fall exactly at free fall speed, but sequential collapse is consistent with the actual rate of fall, which after the first few seconds is obscured by huge clouds of dust.

But in any case, as I think I've said somewhere before, a theory of Controlled Demolition (CD) doesn't solve the problem either. CD allows the process of collapse to begin simultaneously at different levels of a building, but it still relies on gravity to do most of the work of breaking connections, etc. The only process which would enable the entire building to collapse at free fall speeds would be an explosion or explosions which instantly reduced the entire structure to dust, and this is not how CD works.

Hi @DavidB66
Surely the underlying idea is that 'free fall speed' would be incompatible with any simple theory of sequential collapse, which 9/11 conspiracy theorists assume (rightly or wrongly) is the 'official' explanation of the collapse of the Twin Towers (and possibly WTC7, but that gets more complicated). Suppose, say, that Floor 100 and all its contents collapse and fall on top of Floor 99. If the impact force and/or weight of the falling debris is sufficient, the supports of Floor 99 will break and Floor 99 and its contents will also start falling. In the simplest version, the falling debris of Floor 100 will 'gather up' the material of Floor 99, and the combined debris of Floors 100 and 99 will fall together at the same speed until they hit Floor 98, and so on and on, until the debris of the entire building hits the ground. But by basic physics this is incompatible with free fall velocity after the initial collapse of Floor 100.
That is reasonable thinking except there are two key factors you are probably not aware of. And an underpinning necessity for full understanding.
First that "underpinning necessity". Understanding depends on having at least an outline of a qualitative explanation of the collapse mechanism. For the Twin Towers collapses the requirement is to understand that there were TWO main stages of collapse (1) An "initiation stage" which allowed the Top Block to start falling THEN (2) A "progression stage" - the rapid descent to global collapse leaving the derbis heap at or near ground level. The mechanisms of the two stages were very different and conflating the two or ignoring the stage differences has been a cause of much confusion throughout the history of debate of the collapses. A fully reasoned explanation needs two more sub-stages, a total of four, but we don't need that detail at this time.

So the first of the "key factors":
The progression stage is the stage subject of alleged "free fall speed" EXCEPT it wasn't speed. It wasn't free fall << both those are terminology errors. It WAS Free Fall Acceleration. Except it was at about 2/3rds FFA - 2/3rds "G" in conventional scientific terminology. So some form of resistance caused the "loss" of 1/3rd "G"

And the second - the missing 1/3rd "G" is, within the order of accuracy, almost fully explained by changing momentum as the mass of falling debris accumulated.

Now your next comments start to identify some basic aspects of that stage. Let's try these next bits:
is where the rapid motion is subject of discussion and explantion. Energy is needed to break the supports of Floor 99, and all subsequent Floors, and also to communicate a common speed to the combined debris at each stage of he process. Since the only source of this energy (it is assumed) is the kinetic energy of the falling debris, it must be 'paid for' by a reduction in the speed of the falling debris. The reduction may be slight, especially in the later stages of the process when both the velocity and the mass of the falling debris are much higher than at the beginning, but it cannot be zero.
True except for the terminology - not "speed" - it was ABOUT 2/3rds Free Fall Acceleration. And the loss of 1/3rd is almost totally explained by the need to increase and re-balance the momentum of the accumulating falling mass as each floor was stripped off the support and added to the falling debris.

This was possibly the first crude graphic used to explicitly explain the phenomenon in November 2007:

The only significant structural resistance force is the joist-to-column connector labeled "It Fails Here" (and the three analogous ones at the opposite end of the office space joist and both ends of beams in the core area.)
Therefore either the laws of physics or the simple theory of sequential collapse have to be given up.
OR we have a sufficiently accurate reasoned explanation. Of FOUR stages as I stated previously.
I'm no expert on 9/11, so I don't know exactly what the 'official' theory is, but I assume this fairly obvious problem is considered somewhere. So far as I know, it is by and large a theory of sequential collapse, but it does allow for some refinements and qualifications, e.g. to allow for the rotation of large connected pieces of the buildings, different details of collapse for the floors, the core, and the exterior 'walls', etc.
Yes, I and other members can either explain or point you to other explanations.
But I guess (and I'm sure someone will correct me if I'm wrong) the main answer is that the whole building doesn't fall exactly at free fall speed, but sequential collapse is consistent with the actual rate of fall,
Correct.
which after the first few seconds is obscured by huge clouds of dust.
There is sufficient visual evidence in view in the video record for the Twin Towers - WTC7 is different but let's not go there at this time.
But in any case, as I think I've said somewhere before, a theory of Controlled Demolition (CD) doesn't solve the problem either. CD allows the process of collapse to begin simultaneously at different levels of a building, but it still relies on gravity to do most of the work of breaking connections, etc. The only process which would enable the entire building to collapse at free fall speeds would be an explosion or explosions which instantly reduced the entire structure to dust, and this is not how CD works.
Bottom line - no conspiracy theorist or so-called "9/11 Truther" has ever "proved" (presented a valid hypothesis) to show that CD help was needed OR that CD was performed. And no such person has even shown how CD could have been employed to cause the actual collapses seen on 9/11.

So that is my "brief" (???) explanation - more details and fuller reasoning can be provided if you need same.

It demonstrates the collapse of a structure with no vertical resistance.
It does not demonstrate the collapse of any structure, nor is it meant to.

On the right, there's a rope ladder in free fall.
On the left, there's a rope ladder that meets resistance in the form of a table top, and as a consequence falls faster than the ladder on the right.

It does not demonstrate the collapse of any structure, nor is it meant to.

On the right, there's a rope ladder in free fall.
On the left, there's a rope ladder that meets resistance in the form of a table top, and as a consequence falls faster than the ladder on the right.
This seems to allow that an actual structure (like one we might imagine holds up a building) might resist a progressive collapse, slowing it down to less than freefall.

This seems to allow that an actual structure (like one we might imagine holds up a building) might resist a progressive collapse, slowing it down to less than freefall.
It might. But it's not necessarily so, as the experiment shows.

Ultimately, any fall that ends up as a pile on the ground has been slowed to zero.
Un-slowed freefall requires an orbit, or escape velocity.

OR we have a sufficiently accurate reasoned explanation. Of FOUR stages as I stated previously.
Thanks for your very clear explanations. I probably wasn't clear enough myself. The aim of my first para was to set out as strongly as I could why 9/11 'truthers' find the 'free fall' argument persuasive. The argument leads them to the 'either-or' choice I described. My next para pointed out that there are other alternatives, but maybe I didn't stress that enough.

I think part of the problem is that somewhere in the 'official' version (probably the NIST report) there is a passage which is often quoted as accepting that there was a period of essentially free-fall speed (or acceleration - but cumulatively that gives you the speed). Maybe that has been misquoted or misinterpreted.

I'm not sure I understand your statement that

It WAS Free Fall Acceleration. Except it was at about 2/3rds FFA - 2/3rds "G" in conventional scientific terminology. So some form of resistance caused the "loss" of 1/3rd "G"

If something falling near the earth's surface is accelerating at much less than 9 m per sec per sec, as a layman I wouldn't call that 'free fall acceleration'. If there is a technical sense in which that term is applicable I think it should be avoided in communications aimed at a lay audience. A reduction of 1/3 is quite substantial, and gives a clear refutation of the 'truthers'' argument.

I think part of the problem is that somewhere in the 'official' version (probably the NIST report) there is a passage which is often quoted as accepting that there was a period of essentially free-fall speed (or acceleration - but cumulatively that gives you the speed). Maybe that has been misquoted or misinterpreted.
NIST did not speak authoritatively on that phase of the WTC towers collapse.
For WTC 7, a near-freefall phase is acknowledged, but WTC7 did not collapse in the same progressive top-down fashion as the towers.

Suppose, say, that Floor 100 and all its contents collapse and fall on top of Floor 99. If the impact force and/or weight of the falling debris is sufficient, the supports of Floor 99 will break and Floor 99 and its contents will also start falling. In the simplest version, the falling debris of Floor 100 will 'gather up' the material of Floor 99, and the combined debris of Floors 100 and 99 will fall together at the same speed until they hit Floor 98, and so on and on, until the debris of the entire building hits the ground.
That portion happened. There was a clear video of floors blowing out debris, then the next, then the next, etc (occurring very rapidly, until all disappeared in a cloud of destruction), but all that occurred after the initial top collapse.

Thanks for your very clear explanations. I probably wasn't clear enough myself.
Thank you. And "no problem" -- it was the first time we had tried to communicate so both of us learning where the other is coming from.
The aim of my first para was to set out as strongly as I could why 9/11 'truthers' find the 'free fall' argument persuasive. The argument leads them to the 'either-or' choice I described. My next para pointed out that there are other alternatives, but maybe I didn't stress that enough.
Your sentence #1 >> you will find that I'm in some disagreement with many debunkers. (a) I find it perfectly natural that anyone coming uninformed to a first view of WTC Twin Towers collapses would see the resemblance to implosion and would assume CD in the absence of any other explanation AND (b) Not all truth seekers were conspiracy theorists. In my first several years' involvement in these discussions - 2007-8-9 -- a significant number of those seeking truth were genuine, honest truth-seeking laypersons who simply did not have any explanation that they could understand. For the past about 10 years much debunker opinion has polarised into firm denial of both genuine truth-seeking and the reality that not all truth seekers were obsessed conspiracy theorists. It is or rather was perfectly reasonable for people to suspect "CD". Not so reasonable now - 2023. And not been very reasonable since about 2009-10-11-12 when layperson understandable explanations were "published" online.

Your sentence #2 - Yes. And "either-or" is a false dichotomy. There is a valid but more complicated explanation somewhere between the two.

Your sentencer #3 >> no problem - it was a first interaction for both of us.
I think part of the problem is that somewhere in the 'official' version (probably the NIST report) there is a passage which is often quoted as accepting that there was a period of essentially free-fall speed (or acceleration - but cumulatively that gives you the speed). Maybe that has been misquoted or misinterpreted.
Misquoted, misrepresented but also poorly expressed by professionals. Academics and other professionals often do NOT distinguish FF from FFA. They say "free fall", which is a state, when they mean Free Fall Acceleration which is a motion. "They" expect other professionals to know what is intended in the context. For different but related reasons I never rely on any of the Authorities as being always correct or setting the 'gold standard'. The physics stands in its own right on the available evidence. Some of NIST's statements are unclear. A couple are misleading. Whilst the academic "King" - Prof Z Bazant - set the scene for the most influential false understanding and later himself made the same error. And few if any debunkers either identify the errors or are confident enough to point them out. Bottom line - don't rely on NIST, Bazant or others if you can process the evidence for yourself.
I'm not sure I understand your statement that
Mea culpa - my statement is misleading - your correction is better.
If something falling near the earth's surface is accelerating at much less than 9 m per sec per sec, as a layman I wouldn't call that 'free fall acceleration'.
Correct and well-spotted. I must proofread more carefully. My excuse is that I was focused on distinguishing acceleration (the motion) from freefall (the state) and the layperson error of "speed" AKA velocity which isn't Newton's F=ma force-related aspect of the motion. Mea culpa - sorry.
If there is a technical sense in which that term is applicable I think it should be avoided in communications aimed at a lay audience. A reduction of 1/3 is quite substantial, and gives a clear refutation of the 'truthers'' argument.
YES. But, believe it or not, few debunkers actually identify why it is the key. I missed the importance myself for many years. Looked for structural resistances. Never incorporated the momentum dynamics aspects.

That portion happened. There was a clear video of floors blowing out debris, then the next, then the next, etc (occurring very rapidly, until all disappeared in a cloud of destruction), but all that occurred after the initial top collapse.
This is one of the reasons why distinguishing the stages is critical. Many of the significant truther explanations (esp. Szamboti's "Missing Jolt" and Chandler's versions of the same claim) conflate "initiation" and "progression" THEN apply mathematical quantifications which presume continuity across the actual discontinuity. FALSE. The maths is simply invalid irrespective of any other errors. And, sadly, few debunkers spot the errors.

So two stages are essential to avoid those errors and four stages are needed to support comprehensive complete explanations.

I still don't see how this should move a skeptic one way or the other. But if we expect them to learn an important lesson about the WTC from a falling rope ladder, then isn't the skeptic entitled to make their counter-point using a standing rigid ladder?
Argument by example is really only valid if it's a counterexample.

Consider the claim:
All numbers have factors smaller than themselves (and greater than 1).
Then 19 would be a counterexample, proving the claim false.
Of course you'd be "entitled" to point out that 20, 21 and 22 do have factors, but it wouldn't make the claim true.

Now consider the claim:
Free-fall acceleration means there's no resistance
or the equivalent
Resistance means that there can't be free-fall acceleration
That's a truther claim, and the rope ladder experiment is a counterexample proving it false.
However many examples you add, that doesn't make the claim true again.

It's debunked.

P.S.:
No resistance means there's free-fall acceleration
is a different claim (it's true).
Compare:
No calories means no sugar in the food.
That's true. But
No sugar means no calories
is false (butter is a counterexample).
And that's why
Freefall acceleration means no resistance
is false even though the other claim is true.

I think part of the problem is that somewhere in the 'official' version (probably the NIST report) there is a passage which is often quoted as accepting that there was a period of essentially free-fall speed (or acceleration - but cumulatively that gives you the speed). Maybe that has been misquoted or misinterpreted.
Not really misquoted imo:

The analyses of the video (both the estimation of the instant the roofline began to descend and the calculated velocity and acceleration of a point on the roofline) revealed three distinct stages characterizing the 5.4 seconds of collapse:

• Stage 1 (0 to 1.75 seconds): acceleration less than that of gravity (i.e., slower than free fall).
• Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall)
• Stage 3 (4.0 to 5.4 seconds): decreased acceleration, again less than that of gravity
This analysis showed that the 40 percent longer descent time—compared to the 3.9 second free fall time—was due primarily to Stage 1, which corresponded to the buckling of the exterior columns in the lower stories of the north face. During Stage 2, the north face descended essentially in free fall, indicating negligible support from the structure below. This is consistent with the structural analysis model, which showed the exterior columns buckling and losing their capacity to support the loads from the structure above. In Stage 3, the acceleration decreased as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below.

That WTC 7 was in free fall for about 2-2.5 seconds is not really disputed by anyone. But obviously people will argue about what that means, like in this thread here. And truthers like David Chandler will argue that the "Stage 1" of NIST's measurement didn't really happen, it's an artefact of them measuring a kink in the middle of the roofline before the building actually started falling. When Chandler measured a corner rather than the middle, the building goes from being stationary to those 2 second of free fall instantaneously. The Twin Towers also smoothly accelerated down, but only at like 65% of free fall IIRC.

And by the way, nobody's throwing around "free fall" lightly here. It's not that it was just smoothly accelerating down, almost like a dropped object... No, the measurement is 9.8 m/s^2, exactly, for those two seconds.

According to Youtube comments, it seems the answer to the rope ladder is that because the rungs are angled, hitting the table causes them to flatten, which creates a slight downward pull on the above rungs. I'm no expert though, this is just what the comments are saying. But I don't see how it's applicable to the WTC collapses, or specifically WTC 7, since that was the one in free fall.

So if you flattened the rungs, both ladders would fall at the same rate, 9.8 m/s^2. So if we think of the rungs as floors in a skyscraper, that is a bit like what happened with WTC 7 for two seconds. Only problem is that the floors in WTC 7 weren't vertically supported by strings, but by large steel columns. Hmm.

According to Youtube comments, it seems the answer to the rope ladder is that because the rungs are angled, hitting the table causes them to flatten, which creates a slight downward pull on the above rungs. I'm no expert though, this is just what the comments are saying.
It is a valid experiment. Effectively multiple replications of the pivoted lever ball into cup model we have seen many times.
But I don't see how it's applicable to the WTC collapses, or specifically WTC 7, since that was the one in free fall.
WTC 7 was not freefall despite your previous post making 6 or 7 false assertions including one dubious comment by NIST. Remember my caution -don't rely on NIST unless you can check them for accuracy.
So if you flattened the rungs, both ladders would fall at the same rate, 9.8 m/s^2.
Probably true.
So if we think of the rungs as floors in a skyscraper, that is a bit like what happened with WTC 7 for two seconds.
Err...No! It is somewhat similar to the Twin Towers progression stage. Nothing like WTC7. Especially since the topic is Chandler/NIST measured falling of the perimeter shel - AFTER the core structures had already failed.
Only problem is that the floors in WTC 7 weren't vertically supported by strings, but by large steel columns. Hmm.
mmmm.... "strings" == "disconnecting from columns"???? A valid physical analogy??

Surely the underlying idea is that 'free fall speed' would be incompatible with any simple theory of sequential collapse, which 9/11 conspiracy theorists assume (rightly or wrongly) is the 'official' explanation of the collapse of the Twin Towers (and possibly WTC7, but that gets more complicated). Suppose, say, that Floor 100 and all its contents collapse and fall on top of Floor 99. ... But by basic physics this is incompatible with free fall velocity after the initial collapse of Floor 100. Energy is needed to break the supports of Floor 99, and all subsequent Floors, and also to communicate a common speed to the combined debris at each stage of he process. Since the only source of this energy (it is assumed) is the kinetic energy of the falling debris, it must be 'paid for' by a reduction in the speed of the falling debris. The reduction may be slight, especially in the later stages of the process when both the velocity and the mass of the falling debris are much higher than at the beginning, but it cannot be zero. Therefore either the laws of physics or the simple theory of sequential collapse have to be given up.
...
First of all, the experiment in the Opening Post is not relevant to any observation made at the twin towers which collapsed (very roughly speaking) in the top-down fashion you describe.
To that extent, your post is "off-topic".

The experiment applies to an observation made, famously, by Truther David Chandler, who found that some (small) portion of Building 7 exhibited, for a short period late into the collapse sequence, a downward acceleration that averaged the equivalent of "freefall" acceleration. (The five words I bold hint at facts that quaify the assertion that "the building" was "in freefall" with "zero resistance" - it never was in actual fact.)
As a matter of fact, others have found that there is an even briefer time interval where acceleration appears to go above g.

The experiment shows that this is possible, in actual fact without any violation of physics, that portions of a fallling structure can and sometimes do accelerate down at >g.

What Truthers generally forget, what you forgot to account for, and what maybe gets explained too little here at Metabunk:
The "dogma" that energy gets expended on breaking the structure and therefore the building must at all times accelerate at less than g applies strictly only to indeed the entire building (or its full sum of constituing materials, after they came apart), not to mere parts of it.

In the experiment, it is crucial to note that the rungs that hit the table come to a full stop!
And because of this, the entire ladder at all times (beginning with the moment it first touches the table) accelerates at less than g, even while its top rungs accelerate at greater than g.

Same is true with the at/above-g episode of WTC7: While a part of the structure (the north wall roofline) accelerates at or above g, ,other parts of the buidling are decelerated (e.g. by hitting the ground - and that deceleration is IMO part of the reason why the roofline briefly drops at >g! -, and in sum, "the building" (the sum total of its constituents materials; its center of gravity) drops at <g at all times.

I recommend against using this video in debunking efforts. It demonstrates the collapse of a structure with no vertical resistance. That's much more like a building that has been demolished than one that is collapsing under its own weight.
You once more demonstrate your misguided desire to model more than one aspect of a complicated event with the same model.

I advised you harshly in many words to NOT DO THAT! STOP IT!

Do not conflate several aspects of an event into a single model.
Stop wishing for that
You are on a fool's errand.
You are doing modelling wrong.
You absolute MUST knock that stupid idea out of your head, very very hard!

The model demonstrates how one side of a structure hitting resistance (and decelerating greatly) can, in actual fact, make the other side accelerate slightly.

That is all the model demonstrates.
it is NOT intended to demonstrate anything else.
Do not demand for the model to demonstate anything other than what it is intended to demonstrate!
If you make that false and foolish demand, the error is on you.
Stop making such egregious modelling errors by conflating >1 aspect of a complicated event into 1 model.

This seems to allow that an actual structure (like one we might imagine holds up a building) might resist a progressive collapse, slowing it down to less than freefall.
Neither WTC7 nor the model ladder[*] falls at or above g at any time.
Parts of WTC7 move at slightly >g. Parts of the ladder move at slightly >g.
Other parts of WTC7 are very drastically decelerated. Other parts of the ladder are very drastically decelerated.
Therefore, "the building" as well as "the ladder"[*] move at <g at all times.

Do you understand that?

----
[*] once it has started hitting the ground

NIST did not speak authoritatively on that phase of the WTC towers collapse.
For WTC 7, a near-freefall phase is acknowledged, but WTC7 did not collapse in the same progressive top-down fashion as the towers.
Great care must be taken to clarify that at no point ever was WTC7 in a "freefall phase", not even a "near-freefall phase", nor did WTC7 ever fall at (nearly, equal, or even greater than) "freefall-equivalent acceleration".

Only a SMALL PART of WTC7 (part of the North wall) exhibited, for a brief period late into the collapse sequence, such behavior.
At the same time, much larger parts of WTC7 (the core, probably other walls) were in fact decelerated.
In sum, "WTC7" (the sum total of its parts, its center of gravity) was moving at very much LESS than g at all times, including the time frame during which a SMALL PART was observed at g.

This behavior is demonstrated by the Opening Post model: The very sight acceleration of the falling top comes at the expense of the great deceleration of the rungs that hit resistance and come to a full stop in very short time. As a result, the ladder (the sum total of its rungs, its center of gravity, decelerates at all times once it has started hitting resistance below.

So, when (if) "[f]or WTC 7, a near-freefall phase is acknowledged" by NIST, or anyone, that acknowledgement is plainly false! A "near-freefall phase" should only be acknowledged for a minor part of the structure, and for a minor part of the collapse sequence and duration.

I still don't see how this should move a skeptic one way or the other. But if we expect them to learn an important lesson about the WTC from a falling rope ladder, then isn't the skeptic entitled to make their counter-point using a standing rigid ladder?
No, absolutely not. Unless the "skeptic" has made holy vow with themselves to persist in doing their modelling wrong.

At most, the "skeptic" might be "entitled" to make their own counter(?)-point using a FALLING (not "standing"!!!) and NON-RIGID ladder.
For at the time some around-g acceleration was observed briefly for a PART of the structure, that structure was both falling, and it was no longer rigid (the core had already dropped out of sight, and not in synch with the perimeter).

...
Such a ladder would not, I imagine, be vulnerable to total progressive collapse at near-free-fall acceleration.
...
Where do you all of sudden pull this "I want to model total progressive collapse at near-free-fall acceleration" nonsense from?!?

Nothing on 9/11 underwent "total progressive collapse at near-free-fall acceleration".
Nothing.

Not really misquoted imo:

That WTC 7 was in free fall for about 2-2.5 seconds is not really disputed by anyone. ...
FALSE, and the opposite is true.

False, because I can give you a counter-example to this global claim:
I Oystein, dispute it.

Worse: NIST does not actually go quite as far as claiming that (quoting your turn of phrase, Henkka) "WTC 7 was in free fall". What the #32 FAQ talks about is made explicit within the answer NIST provides - you even quote them:
The analyses of the video (both the estimation of the instant the roofline began to descend and the calculated velocity and acceleration of a point on the roofline)
The claim applies NOT to "WTC7", but to "a point on the roofline". I hope you agree without qualification that "a point on the roofline" is something quite different from "WTC7".

(NIST is not fully crystal clear in their wording - but to the extend that they meant to imply, or that Truthers interpret them as implying that "WTC7 was in free fall", that is clearly, unequivocally FALSE, and it matters not who you attribute this falsehood to, and so I dispute it, and you should dispute it, too.)

Nothing on 9/11 underwent "total progressive collapse at near-free-fall acceleration".
Just so we're on the same page. AE911Truth (as many other skeptics) usually cite NIST for this:
“Since the stories below the level of collapse initiation provided little resistance to the tremendous energy released by the falling building mass, the building section above came down essentially in free fall, as seen in videos.” — p. 146, NIST NCSTAR 1
https://www.ae911truth.org/evidence/near-free-fall-acceleration

Just so we're on the same page. AE911Truth (as many other skeptics) usually cite NIST for this:

https://www.ae911truth.org/evidence/near-free-fall-acceleration
How many times do you need warning? Do not be misled by NIST's loose usage of language especially where the issue is confusing Free Fall and Free Fall acceleration. No matter how many other 'sceptics' cite the dubious explanation.

How many times do you need warning? Do not be misled by NIST's loose usage of language especially where the issue is confusing Free Fall and Free Fall acceleration.
I think everyone involved means it in this loose sense.

According to Youtube comments, it seems the answer to the rope ladder is that because the rungs are angled, hitting the table causes them to flatten, which creates a slight downward pull on the above rungs.
No, it does not matter at all. whether the rungs are horizontal or at some (small) angle away from horizontal.
What matters is that only one side of the rung hits resistance.
You could get the same effect with a ladder of horizontal rungs, but instead of a flat ground, it falls onto a couple of massive poles in such a way that each impact chips away a bit of the impacted pole, and so they are hit alternatingly.

I'm no expert though, this is just what the comments are saying. But I don't see how it's applicable to the WTC collapses, or specifically WTC 7, since that was the one in free fall.
At the WTC7, you have resistance to "rungs" only on one side.
Think of a ladder: The left upright is the core of WTC7, the right upright is the perimeter, and the rungs are the floor beams spanning between core and perimeter.
You know, don't you, that the core started descending some fraction of a second before the perimeter did, right?
That's, obviously, because the core columns somehow lost support somewhere below.
So, in our model, that could mean we hold the ladder up from the ground a bit and let the left upright drop.
But a normal ladder as we have them at home is pretty rigid - you remove support on one upright only, then that upright would not drop, the rungs are too firmly attached on both sides and would hold it up.
So, for a fair model, we need to modify that ladder a bit:
I hope you agree that probably many floor beams did not detach immediately, yet the core dropped. That means the floor beams can be bent somehow, either along their length, or they can pivot a bit about their connections on either side. Or a bit of both. So lets make our ladder with rubber rungs, or attache them to the uprights such that they can pivot about the connection, or both.

Can you picture this, or need I draw a sketch, or build somemthing for you?

Ok, so, lets hold the ladder up. Then let the left upright ("core") drop.
Then, a fraction of a second later, let's also drop the right upright ("perimeter").
make it so that, for a moment, the entire ladder falls.
What happens next?

-> The left upright, because it started falling earlier, will hit the ground before the right upright does. It will be greatly decelerated and come to a full stop in very short time.
This upwards resistance is instantly (at the speed of sound) communicated up to our somewhat flexible rungs.

The rungs will react like the pencil does in my experiment, or indeed like the bottom rung does in the OP experiment: Go into rotation with the opposite end psuhing DOWN (in the opposite direction of the resistance encountered).

And that will give the right upright an impulse DOWN.
it will will accelerate at >g.

And that's how @Thomas B ought to model his "sleptic's ladder": As a FALLING and NON-rigid ladder. Because that is what the system core - floor beams - North wall was at the WCT7: a FALLING and NON-rigid ladder, whose core-side vertical support ran into the ground before the other side did; and that deceleration experienced by the core hitting ground translated, via rotating floor beams, into a DOWNward acceleration of the North wall columns. This pushed a North wall that had some residual resistance and would move at somewhat less than g to g, or a North wall that had completely lost structural support to >g.

So if you flattened the rungs, both ladders would fall at the same rate, 9.8 m/s^2. So if we think of the rungs as floors in a skyscraper, that is a bit like what happened with WTC 7 for two seconds.
No. Wrong.
Because the floor beams (rungs) of WTC7 were not horizontal when this "about g" episode occurred. They were at a slight angle to horizontal because the core had begun to fall before the perimeter did.

I think everyone involved means it in this loose sense.
No.
Need counter-examples to debunk this global claim? I have (at least) 2 such counter-examples. Can you guess who I am thinking of?
And my advise is for you to NOT employ "loose sense" of language if you really want to understand and overcome your confusion.

Just so we're on the same page. AE911Truth (as many other skeptics) usually cite NIST for this:

https://www.ae911truth.org/evidence/near-free-fall-acceleration
If you mean to say that this sentence by NIST is wrong, then yes, we are on the same page.

Provided your reason for saying so is that you understand and admit that WTC7 was not at any time, not even during the short time interval when a point on the roofline was observed to move at about g, anywhere near freefall acceleration.

Just as "the ladder" in the OP video ceased being in freefall and continued moving at LESS than g the moment its first rung hit the ground.

Only a SMALL PART of WTC7 (part of the North wall) exhibited, for a brief period late into the collapse sequence, such behavior.
You're splitting hairs... This is what we're talking about:

The roofline starts to descend about 10 seconds into the video. For the first 100 feet of the fall, the roofline is going at 9.8 m/s^2. This lasts for about two seconds. To describe what everyone can plainly see in the video as "Only a SMALL PART of WTC7" feels like gaslighting and feels like you're being overly defensive. Like the debunker doth protest too much. Sure it's the north wall, but that's because that's the only wall we can see in the video! Well, we can see the northwest wall also, and it's falling just the same. There's no reason to think the walls we can't see are doing something radically different.

Just so we're on the same page. AE911Truth (as many other skeptics) usually cite NIST for this:
“Since the stories below the level of collapse initiation provided little resistance to the tremendous energy released by the falling building mass, the building section above came down essentially in free fall, as seen in videos.” — p. 146, NIST NCSTAR 1
Content from External Source
https://www.ae911truth.org/evidence/near-free-fall-acceleration
Yes, but that's a) loose language ("essentially"), and b) non-authoritative, i.e. NIST cites/discusses no evidence to support this statement, and does not quantify it (no numbers). They did not study this phase of the collapse.

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