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Freefall Speed
There are various arguments revolving around the phrase "freefall speed" or "freefall acceleration." The general argument is that the buildings fell at or near freefall acceleration and thus could not be natural collapses.
Table of contentsFreefall Speeds?Did any of the three WTC Buildings collapse at freefall speeds? No.The term is a bad one. Speed, or velocity, is a measurement of how fast an object is moving at any one point in time. Acceleration is a measurement of how an object's speed is changing over time at any one point in time. Free fall is a condition that describes acceleration, not speed. So if you ask how fast some object is moving, that is speed. If you ask how much an object is speeding up or slowing down, that is acceleration. And free fall is a special category of acceleration: it is the rate of acceleration an object has if one source of gravity is the only force operating on it. Free Fall AccelerationDid any of the three WTC buildings collapse at free fall acceleration? Only WTC 7 did, and that was only for 2.25 seconds of its 16+ second collapse. See below for a discussion of this period.Pictures of the Towers clearly show debris falling separately from the main tower. Separate from the main structure, the only forces operating on those pieces are air resistance (minimal) and gravity. The debris descends at a rate of acceleration much greater than that of the main portion of the collapse.  Perimeter columns from the South Tower are on the left. Perimeter columns from the North are on the right. You can see that these pieces are falling faster than the actual collapse, because they've fallen below where they broke off. You can see the undamaged face of the South Tower to the extreme left and the undamaged face of the North Tower to the extreme right. I've prepared a short video showing that 7 World Trade took at least 16 seconds to fall completely, with seismic information indicating 18 seconds. However, CD advocates are usually referring to a single portion of the entire collapse, when the roofline begins to fall. The Final Report on 7 World Trade from NIST demonstrates that this section fell 40% slower than it would have if moving at freefall speeds. AE911Truth has graphed out the descent of this final portion to have a 9.1 m/s-squared constant. Gravity accelerates at 9.8 m/s-squared. Even this portion of the building does not descend at free fall acceleration. This graphic is currently found on the AE911Truth slide 71 of the full presentation. "Near" Freefall Speeds?Did any of the three WTC buildings fall at "near" freefall speeds or acceleration?That all depends on how "near" is defined. An acceleration rate of 9.1 m/s-squared is nearer to 9.8 than 8.7, but it's further away than 9.7. Trying to define "near" in this case is a fool's game. The more appropriate question to ask is, "How fast should these buildings have fallen?" If mathematical analysis takes all appropriate factors into account and shows the buildings fell within an expected time range, then it wouldn't matter if the buildings fell at free fall acceleration, near free fall acceleration, or much slower than free fall acceleration. There would be nothing unusual about the time at all. If, however, the actual acceleration is outside this range, that would indicate a significant factor that had not been accommodated, one of which could conceivably be controlled demolition. Such analysis has been done. As NIST explains: ...the momentum (which equals mass times velocity) of the 12 to 28 stories (WTC 1 and WTC 2, respectively) falling on the supporting structure below (which was designed to support only the static weight of the floors above and not any dynamic effects due to the downward momentum) so greatly exceeded the strength capacity of the structure below that it (the structure below) was unable to stop or even to slow the falling mass. The downward momentum felt by each successive lower floor was even larger due to the increasing mass. The section of 7 World Trade in the final part of the collapse is quite similar in mass to the upper sections of the towers. More rigorous calculations have been published in respected scientific journals (Bazant & Zhou (pdf), and Bazant, Le, Greening, and Benson (pdf) (in press)). The results work out to the same conclusion. The dynamic loads of the descending mass was far too large to be redistributed by any structure available to resist them. The acceleration of their descent was not mathematically unusual. Therefore, the buildings falling at speeds that can be described as "near free fall acceleration," whatever that vague term means, isn't something that rules out the buildings falling naturally. AE911Truth's ArgumentsNo Change in Timing?The argument made by AE911Truth about freefall speed has changed in the case of 7 World Trade. Their first couple of presentations described 7 as falling at freefall speed, period. That has changed. Now they describe 7 as falling at "nearly freefall speed" just as they always have the towers.However, they continue to say that 7 World Trade fell in 6.5 seconds. You can still see Gage make this claim in interviews available on their front page, and a graphic detailing this claim appears at the link to their "WTC 7 Evidence" page in this paragraph. However, assuming that their graphic does count off the seconds accurately, the building clearly has one-third of its height left to fall when AE911Truth stops the count. It's unusual to me that they still claim the building to have fallen in 6.5 seconds but have changed their statement from "Freefall Speed" to "Near Freefall Speed." Wouldn't they be obliged to change this 6.5 second count if they think the building took longer to fall? Or were they being contradictory before? Regardless, they now claim that all the buildings fell at near freefall speed. As I've explained above, the time it took for all three of these buildings to fall isn't outside an expected range mathematically. As shocking as it may be to our inexperience in large buildings falling, this is how quickly buildings of this size fall when they begin to collapse. Update: with the ascent of David Chandler to the stage, the characteristic features list on AE911Truth's front page has now returned to saying that WTC 7 fell at freefall speeds. And yet their slide 73 still says "Nearly Freefall." Clearly there is no competent editorial hand at AE911Truth. See below for the Chandler videos. Further update (5/31/2010): AE911Truth is to be commended for excising the term "freefall speed" from their front page. Now they need to do so within their PowerPoint? presentation. 7 Faster and Slower?Funnily enough, AE911Truth currently argues that 7 World Trade descended both slightly faster than and slower than the acceleration of gravity!They've just added a video to their presentation in which someone takes the Dan Rather video and tries to very precisely measure the fall of the building. They conclude that the building fell at an acceleration rate of 10 m/s-squared, which is faster than gravity accelerates! This can be found on Slides 69-70. Then on slide 71, they repeat their argument (seen above) that the building fell at a rate of 9.1 m/s-squared! Furthermore, the 10 m/s-squared figure comes from just the first 3 seconds. They don't provide any more data past that. Now I'd love to argue that this section falls quickly in the first part and then slows down enough due to resistance to make the 9.1 figure the average speed of descent. But I can't. The AE911Truth graphs says that the acceleration rate is constant for the building. There is no marked change in the rate of acceleration. So I am left scratching my head at the AE911Truth presentation. They genuinely are arguing right now that the building fell at two different acceleration rates. Using their own figures and graphs, you can see that they don't have a clue of what they're talking about. NIST's Timing of CollapsesAnother argument you will frequently see about the collapses of the towers is that "NIST claims the towers fell in 9 seconds and 11 seconds." However, a look at the report and a FAQ list published by them shows that NIST's figures are timing from the beginning of the collapses to the time the first amount of debris hit the ground. It's natural that this time would be quite close to a time free fall acceleration would produce.NIST estimated the elapsed times for the first exterior panels to strike the ground after the collapse initiated in each of the towers to be approximately 11 seconds for WTC 1 and approximately 9 seconds for WTC 2. These elapsed times were based on: (1) precise timing of the initiation of collapse from video evidence, and (2) ground motion (seismic) signals recorded at Palisades, N.Y., that also were precisely time-calibrated for wave transmission times from lower Manhattan (see NCSTAR 1-5A). As documented in Section 6.14.4 of NIST NCSTAR 1, these collapse times show that: … the structure below the level of collapse initiation offered minimal resistance to the falling building mass at and above the impact zone. The potential energy released by the downward movement of the large building mass far exceeded the capacity of the intact structure below to absorb that energy through energy of deformation. 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. As the stories below sequentially failed, the falling mass increased, further increasing the demand on the floors below, which were unable to arrest the moving mass. In other words, the momentum (which equals mass times velocity) of the 12 to 28 stories (WTC 1 and WTC 2, respectively) falling on the supporting structure below (which was designed to support only the static weight of the floors above and not any dynamic effects due to the downward momentum) so greatly exceeded the strength capacity of the structure below that it (the structure below) was unable to stop or even to slow the falling mass. The downward momentum felt by each successive lower floor was even larger due to the increasing mass. From video evidence, significant portions of the cores of both buildings (roughly 60 stories of WTC 1 and 40 stories of WTC 2) are known to have stood 15 to 25 seconds after collapse initiation before they, too, began to collapse. Neither the duration of the seismic records nor video evidence (due to obstruction of view caused by debris clouds) are reliable indicators of the total time it took for each building to collapse completely. As NIST points out, sections of both cores remained standing after the collapse, and any time of total collapse should include the time it took for these structures to fall as well. There's just no way to get a handle on a precise time for total collapse of these buildings. AE911Truth's attempt to do so shows their unscientific methods of investigation. The Chandler Physics VideosDavid Chandler is a high school physics teacher and contributing member of AE911Truth. He has taken a video of WTC 7 falling and identified a short period of time where the building appears to fall at free fall acceleration. He then accuses NIST of covering up this period by assuming a constant speed for this collapse.This attack is based on a draft version of NIST's final report on WTC 7. Let's look at what they did. This is from pages 40-41 of the draft report: NIST was interested in estimating how closely the time for WTC 7 took to fall compared to the descent time if the building were falling freely under the force of gravity (NIST NCSTAR1-9, Chapter 12). Assuming that the descent speed was approximately constant, the two quantities needed for the determinations were (1) a length that some feature of the building descended and (2) the time it took to fall that distance. The chosen feature was the top of the parapet wall on the roofline of the north face. The length was was the difference between the position of the roofline prior to the collapse and the last position the roofline could be observed before it was obstructed by a building in the foreground. ...the actual time for the upper 18 stories to collapse, based on video evidence, was approximately 40 percent longer than the computed free fall time and was consistent with physical principles. Mr. Chandler took a much grainier version of the same video, loaded it into a computer program, and did as precise a job as he could of measuring the parapet wall's descent frame by frame. The program he used then could tell him the rate of acceleration at any given moment. This is the chart he uses in his video: As you can see, Chandler discovers an approximate time of 2.5 seconds where the building appears to be in free fall. Of course there are other places where the building is descending at the same rate of velocity, meaning that it is not speeding up at all, as it would under free fall conditions. But Chandler only focuses on this 2.5 second period. In fact, he stops his charting well before the building disappears from view. Since that last half second shows no acceleration at all, one would expect to see a lot more resistance to the collapse as it continued. Chandler has not disproved NIST's calculation at all. In fact, he's underscored it. Even with this period of free fall, the building itself fell behind the obstruction 40% slower than it would have if the entire building had been subject to free fall speeds throughout its collapse. The building experienced significant resistance as it fell. Chandler then pretends that NIST is trying to cover up this section of the collapse. Silly accusations like this don't serve the cause of truth at all. He would have done better to see if anything else in the draft report could have accounted for these 2.5 seconds. And there is. Figure 3-14 in the draft report shows Floors 7-14 buckling due to the way the collapse would have propagated. It is this buckling of eight floors that is seen when the building drops down in free fall acceleration for this short part of the collapse. And NIST realized this, because the final version of this section repeats the initial calculations and then measures much more precisely the complete descent. They found that there was actually a 2.25 second period of virtual free fall, and at the rate it was falling, it would have traveled approximately eight floors during those 2.25. That would be Floors 7-14, precisely. So far from displaying alleged chicanery by NIST, David Chandler has only helped to confirm their computer modeling with visual analysis.
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