Free-Fall Acceleration -World Trade Center Building 7

The National Institute of Standards and Technology (NIST) acknowledges that WTC 7 fell at a rate of free fall (or the rate of gravity) for a period of approximately 2.25 seconds before it started to slow down. David Chandler, a physics teacher who has studied the behavior of WTC 7 extensively, explains the significance of free fall in the article titled “Free Fall and Building 7 on 9/11.

“Newton’s third law says that when objects interact, they always exert equal and opposite forces on each other. Therefore, while an object is falling, if it exerts any force on objects in its path, those objects must push back, slowing the fall. If an object is observed to be in free fall, we can conclude that nothing in the path exerts a force to slow it down….”

Applying this to WTC 7, he explains:

“Free 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….”

Chandler and others therefore interpret WTC 7’s free fall as evidence of controlled demolition. How does NIST explain the occurrence of free fall according to its hypothesis of fire-induced failure? To answer that question satisfactorily, we must first examine NIST’s initial attempt to deny the occurrence of free fall.

NIST’s Denial of Free Fall
On August 21, 2008 — six years to the day after NIST’s World Trade Center investigation was first announced — NIST released its draft report on WTC 7 for public comment. In it, NIST described the collapse time of WTC 7 as being 40 percent longer than the time it would take to collapse in free fall:

“The time the roofline took to fall 18 stories was 5.4 s[econds]…. Thus, the actual time for the upper 18 floors of the north face to collapse, based on video evidence, was approximately 40 percent longer than the computed free fall time….” 3

NIST repeated this claim in its Questions and Answers about the NIST WTC 7 Investigation (WTC 7 FAQs), stating unequivocally, “WTC 7 did not enter free fall.” NIST’s lead investigator, Dr. Shyam Sunder, repeated it again at NIST’s WTC 7 Technical Briefing on August 26, 2008, when asked the following question, which had been submitted by David Chandler:

Chandler: “Any number of competent measurements using a variety of methods indicate the northwest corner of WTC 7 fell with an acceleration within a few percent of the acceleration of gravity. Yet your report contradicts this, claiming 40 percent slower than free fall, based on a single data point. How can such a publicly visible, easily measurable quantity be set aside?”

Dr. Sunder responded by articulating the meaning of free fall in the clearest terms possible, but denied that is what happened in the case of WTC 7:

Sunder: “[A] free-fall time would be an object that has no structural components below it…. What the analysis shows…is that same time it took for the structural model to come down…is 5.4 seconds. It’s about 1.5 seconds, or roughly 40 percent, more time for that free fall to happen. And that is not at all unusual because there was structural resistance that was provided in this particular case.”


World Trade Center 7: NIST Finally Admits Freefall by David Chandler