![]() But of course, a linear regression is easy to perform… Also, the errors of these data points are not independent. You do the regression on virtual points (the start at 1.75s and the end at 4s are not even moments at which we have measured anything). In other words, for every two neighbouring points in Figure 12-76 they took the velocity of the moment halfway as if the velocity was constant between those two points! This is a questionable approach, I think, as this loses information. They are calculated by using a central difference approximation. Those velocity data points, however, are not directly measured velocities. In this way, they find an acceleration of 32.196 ft/s 2 (= 9.81 m/s 2) which is pretty close to the gravitational acceleration g indeed. Now they assume that Stage 2 has a constant acceleration and approximate this by performing a linear regression on the velocity data points. Without much ado, NIST tells us that they see three stages by cutting this 5.4-second period (during which the fall of the North facade is visible) at 1.75s and 4s: “The slope of the velocity curve is approximately constant between about 1.75 s and 4.0 s.” Just by eyeballing the graph it seems, and the cuts seem quite arbitrary to me. They use the derivative of this curve (as an approximation of the actual speed) to identify the three stages. NIST fitted a curve through these points using a formula that does not really have physical meaning. The data points are given in the following graph: How they did this exactly and how accurate that was done is also not entirely clear. Like Chandler, NIST picked a point from a video (a different one than Chandler used) showing the collapse, followed that point going down and measured time and height. ![]() You can download the full report or just the pages that I will be talking about. NIST explains what they did in paragraph 12.5.3 of the final report. Although I had some doubts about the approach NIST used to calculate the acceleration, I didn’t really have a close look and limited my criticism to pointing out that NIST didn’t show how accurate their calculation is and that the identification of the three phases seems a bit arbitrary. ![]() Occasionally this issue has come up in discussions I had with people who are quite sceptical about the official explanation for the collapse of WTC7. If you’re interested, you could watch part 2 and 3 as well. Let’s see how this came about from the viewpoint of David Chandler, who showed that there seems indeed to be a short period of time during which the northern facade of the building came down at an acceleration equal to the free fall acceleration (9,81 m/s 2). ![]() But they also wrote that this was still “consistent with the results of the global collapse analysis” (for a discussion on whether that makes sense I refer the reader to Metabunk). After questions were raised they had another look and in the final report they acknowledged that for a period of 2.25 seconds (within the 5.4 seconds) there appeared to be acceleration “equivalent to the acceleration of gravity g“. In their preliminary report NIST had stated that based on the visible evidence it had found that when the exterior facade came down, 18 floors descended in 5.4 seconds, implying an acceleration about 40% lower than free fall acceleration. ![]() Truthers claim that this cannot be explained by the NIST models, but only by controlled demolition (even though usual controlled demolition doesn’t show free fall for several seconds either). One argument that has dominated the discussion is ‘the fact’ that WTC7 came down at free fall speed. Truthers dispute the official explanation, given by NIST, that the building collapsed after fires had raged uncontrolled over several floors during the day after the building was hit by debris from the North Tower. The collapse of this building has become one of the main issues that truthers see as proof that the official story is fabricated by the US government to conceal that the buildings were in fact brought down with explosives in a controlled demolition. Another huge building, WTC7, came down later that day. On 9/11 not only the two main towers of the World Trade Center collapsed after terrorists flew planes into the buildings. ![]()
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