Free Fall — Part 3

by David Chandler

David Chandler
9 min readApr 21, 2018


Part 1 2 3 4 5 6

In Part 1 we looked at the kinematics (analysis of motion) of the fall of WTC 7. We reviewed the process of measuring acceleration and saw the direct observational evidence that the NW corner of World Trade Center Building 7 (WTC 7) descended at the acceleration of gravity for the first 2.5 seconds of its fall.

In Part 2 we looked at the dynamics (analysis of forces) of the fall of WTC 7. We established that as a direct consequence of free fall the falling section of the building could not have been what was crushing or removing the underlying structure. The building had to have been falling as a consequence of structure that was removed by other forces.

At this point it is easy to see why many scientists, engineers, and architects see pre-planted explosives as the only plausible explanation of the forensic evidence, acknowledging that there is a very deep rabbit hole of logical consequences that follow. The official story is all about avoiding that rabbit hole, but any honest investigation would need to do justice to the forensic evidence. So let’s see how the official investigation handled the free fall question.

The Bush 43 administration handed the task of explaining the collapses of the World Trade Center buildings to NIST, the National Institute of Standards and Technology, an agency in the Department of Commerce in the executive branch of the government. It is not an independent agency. NIST released their first report on the Twin Towers without mentioning WTC 7 in 2005. It discussed the collapse of WTC 7 in a separate report released in 2008, at the very end of the Bush 43 presidency.

We will take the WTC 7 story in two parts. NIST issued their Final Draft for Public Comment in August 2008. I and others responded to this report, both in writing (in a formal “Request for Correction”) and through questions at a video taped Technical Briefing Conference. Changes were made in response to these comments and the final report was issued in November 2008. We will focus here on the August report, and discuss some of the objections that were raised, and cover the final report in Part 4.

NIST’s Final Report on the Collapse of World Trade Center Building 7; Draft for Public Comment is divided into a shorter summary report labelled NIST NCSTAR 1A and the full report labelled NIST NCSTAR 1–9. Section 3.6 of the summary report, starting on p. 40, discusses “Collapse Time.”

The section starts by saying, “NIST was interested in estimating how closely the time for WTC 7 took to fall compared with the descent time if the building were falling freely under the force of gravity.” It is curious that the framing is in terms of time rather than “acceleration.” The realistic task at hand was to correctly characterize the collapse of the building. The overall time of fall is not the issue. The most natural and meaningful way to compare the fall of the building with the acceleration of gravity would be to measure the acceleration of the building. NIST, in this report, chose not to do this. In the very next sentence they spell out their alternative methodology: “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 reference to “constant speed” was clearly an error. Even casual observation indicates that the building was not falling at constant speed, and that while it was moving downward it was accelerating. It is clear from context that the sentence was confused because the following paragraph starts, “The theoretical time for free fall (i.e., neglecting air friction), was computed from, t = √(2h/g),…” This formula applies only to motion at the acceleration of gravity, starting at rest, with no resistance of any kind. This approach is equivalent to using a stop watch to record the starting and ending time. Use of this formula completely contradicts the constant speed assumption.

Even if the report had said, “Assuming that the acceleration was approximately constant,” the methodology would still be invalid because there is no a priori basis for that assumption. We covered this issue in Part 1 of this series. The only way to determine the acceleration of the motion in a complex situation (such as a building falling through its own structure) is to compute the velocity as a function of time, over many short intervals, and determine the rate of change of the velocity from one interval to the next. Typically one would accomplish this by graphing velocity vs. time and computing the slope in any interval of interest. The virtual stopwatch approach, employed by NIST, is equivalent to computing the slope of a straight line connecting the first and last data points on the graph, ignoring all the data in between. This approach provides a meaningless result that obscures the true motion of the building. If NIST had provided the actual graph of velocity vs. time, and connected the first and last dots with a straight line, we could see for ourselves that the virtual stopwatch approach was a ludicrous irrelevancy. The actual motion of the NW corner of the roof, over the 5.4 second time interval chosen by NIST, was to remain at rest for about 1.5 seconds, then fall at the acceleration of gravity for about 2.5 seconds, and then continue accelerating downward at less than free fall over the remaining time. To do a simple stopwatch timing to characterize a motion of this complexity is a completely meaningless exercise. Coming from a professionally competent scientific organization such as NIST, it is hard to see this analysis as anything other than an attempt at deception.

As we stated above, the gravitational acceleration formula stated in the report applies only if there is zero resistance and only if the start time is the actual starting time of the fall. So what about NIST’s choice for the start time for their virtual stopwatch? The report states that the start time was the very first indication of motion of the roof line. In the technical briefing conference that followed the release of the August draft report, John Gross spelled out explicitly that they looked for a single pixel change from the color of the building to the color of the sky near the middle of the roof line to mark the start of downward motion. However, the report also states that they used Camera 3, which looks up at an angle at WTC 7 from ground level on West Street. From this angle it would be impossible to distinguish vertical motion from lateral motion of the roof line.

Fully developed “kink” as seen from Camera 3, looking up from ground level on West Street. This kink is not visible from vantage points level with the roof line, so it is clearly a lateral fold rather than a vertical kink.

In fact, the motion NIST designated as the start of downward motion was actually a lateral motion. From the perspective of the upward-looking camera, a V-shaped kink seemed to develop in the roof line, but in fact it was not a vertical kink but a lateral fold along a vertical axis. We know this because other cameras positioned level with the top of the building, show no kink. The roof line remains straight, right up to and through the time of global collapse. NIST knew the fold was lateral because they say so. On p. 275 of the full version of the report (NIST NCSTAR 1–9) the kink is described by saying, “It also appears that the east end of the north face is rotating to the north relative to the rest of the north face.” So relying on the inherently ambiguous geometry of the upward-looking camera angle to misrepresent what they knew to be lateral motion, and treating it as the start of downward motion, again appears to be an attempt at deception. The outcome was a starting time that was about a second and a half before the actual onset of downward motion. From NIST’s start time to the time when the roof line descended to the 29th floor (the lowest point visible from Camera 3) was 5.4 seconds. Free fall to the 29th floor would have taken about 3.9 seconds, so if the extra 1.5 seconds were omitted, all we would have left would be free fall transitioning to near free fall. NIST clearly didn’t want to go there.

The 5.4 second interval with a clock added. Notice the state of the building when NIST started their virtual stopwatch. Also notice the lack of a vertical kink in the roof line.

I have created a video with a clock running at one revolution per second. The end of the timing interval is when the roof line is at the 29th floor. When the clock is run backward from this point for 5.4 seconds you can see that when the clock was started the roof line was not descending at all. John Gross’s changed pixel had to have been the result of the horizontal motion of the roof line.

NIST’s explanation of the collapse of WTC 7 in Section 3.6 is sufficiently incoherent that one might wonder what it actually says. On the surface it appears to be a claim that the building fell significantly slower than it would have if it were in free fall. But NIST never actually commented on the acceleration of the building. They simply compared two unrelated time intervals: 1. the time from the first pixel color change until the roof line reached the level of the 29th floor, on the one hand, and 2. the time for an object in absolute free fall to traverse the same vertical distance, on the other. One might claim (if one were a lawyer) that NIST made no claim about the actual acceleration of the building at all. It is only by glossing over the incoherence of the argument that we think NIST has claimed that the building fell slower than free fall. Scientists and engineers don’t communicate this way, which is why I am led to speculate that this section may well have been written in collaboration with a legal team. NIST desperately wanted to deny free fall, because that is the opening of a very deep and troubling rabbit hole. But they didn’t want to actually lie, in the sense of producing a document that would be provably fraudulent in court. So, according to my best guess, NIST opted for incoherence to avoid an overt lie.

NIST’s account gets more tangled, however, as we go on. At NIST’s Technical Briefing Conference during the period for public comment I was able to submit a question.

Discussion of free fall, NIST Technical Briefing, August 2008

I referred to the measurable acceleration of the building as being (generously) within a few percent of free fall compared with NIST’s claim that it fell 40% slower than free fall. I asked, “How can such a publicly visible, easily measurable quantity be set aside?” Shyam Sunder, the lead investigator for NIST’s WTC 7 report responded, “A free fall time would be an object that has no structural components below it….” But, “there was structural resistance that was provided in this particular case, and you had a sequence of structural failures that had to take place, and everything was not instantaneous.” In other words, even though the report arguably avoids the outright lie of denial of free fall, through a (seeming) strategy of incoherence, Shyam Sunder fills in the gap here and provides the outright denial that the building was in free fall, because, as he explains, it could not have been in free fall. Rather than try to explain the observable reality of free fall, NIST attempted to obscure the fact of free fall using an invalid, and completely meaningless measurement and computation. Those of us who addressed free fall in the period for public comment apparently forced a reconsideration of this strategy because in the final report NIST changed tactics.

I don’t want to leave this topic without suggesting how the change in the final report may have been brought about. Later in the Technical Briefing Steven Jones, a physicist at BYU, pointed out the simple and undeniable error in the report: “NIST discusses the fall time for WTC 7 on page 40 of the summary report, where it’s stated, ‘assuming that the descent speed was approximately constant.’ However observations by others of the descent speed show that the building is accelerating rather than being at constant speed. Why did NIST assume that the descent speed was approximately constant?”

Shyam Sunder handed that question to John Gross, who spent about 45 seconds recounting how the measurement was made and saying absolutely nothing in response to the question, while fidgeting with his pen, rubbing his nose, and displaying extreme discomfort. At that point Shyam Sunder asked if someone could clarify the comment. Someone else leaned in and said they would have to correct this in the final report.

With that response we had a commitment, documented on video, to change the final report. The door had been opened. One might think a simple change of wording in that one spot might have fulfilled the commitment, but as we shall see in Part 4, NIST responded with a new analysis that actually acknowledged free fall. Isn’t that interesting! More to come.

Part 1 2 3 4 5 6



David Chandler

BS physics/MA education/MS math; retired from ~35 years teaching physics, math, & astronomy in high school and college.