Free Fall — Part 5

by David Chandler

David Chandler
8 min readMay 13, 2018


Part 1 2 3 4 5 6

We have been talking exclusively, up until now, about the free fall of WTC 7. But what about the Twin Towers? Of the two, the simpler case is the North Tower (also known as WTC 1).

WTC 1, a 110 story building, was hit by a Boeing 767 at the level of the floor slab of the 96th floor. The fuselage split and entered the 95th and 96th floors. The wings were tipped to the left, so varying degrees of damage extended from the 93rd to the 99th floor.

The official story evolved over time. The public was presented, very early on, with the “pancake collapse” theory in the NOVA program, “Why the Towers Fell” in May 2002, and the notion stuck in the public mind.

“But the tall vertical columns of the inner core and the outer walls were like free-standing stilts until Robertson tied them together with floor trusses.” — NOVA narrator

The pancake theory, and the illustrations used in the NOVA program, drastically understated the core structure, which actually consisted of 47 massive interconnected columns gathered together near the center of the building, acting as a very rigid building within a building.

If the floors pancaked, the core would remain as a free-standing tower, when all was said and done. This clearly did not happen. NIST rejected the pancake theory and instead postulated that the upper section of the building acted as a “pile driver” that crushed the columns of the lower section of the building, causing them to buckle. They relied on the work of a structural engineer from Northwestern University, Zdenek Bazant, who envisioned the top block crushing the lower block to the ground before crushing itself in the end: crush down followed by crush up, as he put it.

(Bazant’s paper was submitted for publication two days after the events of 9/11.) There are many things wrong with this analysis, but NIST adopted it without critique as a substitute for doing their own analysis.

Turning to observations, the first major ejection of dust and debris marking the start of collapse occurred at the 98th floor, so if we were to view the collapse as consisting of a top block of floors falling and crashing against a lower block of floors, it would be 12 stories vs. 97 stories. The top 12 stories could not survive to crush the lower 97 stories because the forces of interaction would be equal and opposite at every point. The two sections would destroy each other at the same rate, so by the time the top 12 floors of the bottom section were destroyed, the top section would also be destroyed, leaving nothing to crush down the rest of the building.

I say “if we were” to view it this way because if you watch the start of the collapse, the roof actually moves downward to about the level of the 105th floor before the top “block” starts its descent at all. In other words the top block does not act as a coherent unit that might be viewed as a hammer, or pile driver, that could crush the columns of the lower stories. It is nearly 50% demolished internally before it starts moving into the lower section of the building. NIST provides no insight into how the top section, with no pile driver above it, might accomplish this self-demolition.

We could stop there and say NIST failed to describe the reality, but let’s continue, for the sake of argument, by granting their assumption that the top block acted as a coherent unit capable of crushing what was below it. What insight might be gained by measuring the motion of the roof line? Is it, for instance, in free fall?

I measured the downward motion of the roofline and the answer is no, it was not in freefall. It fell with a uniform downward acceleration about 64% of the acceleration of gravity.

So what are the implications of that result? Unlike the simple case of free fall, we actually have to do some math in this case. I started by wondering to myself, what does this less than free fall downward acceleration say about the force of interaction between the upper and lower blocks?

To answer this question I started with the kind of analysis I teach my students in introductory physics classes. I drew a free body diagram.

This kind of diagram isolates a single object (the top section of the building in this case), upon which you represent all the forces acting on it, then combine the forces to find the net force. Finally you apply Newton’s Second Law which says that the net force equals the mass times the acceleration of the object.

In this diagram we see that the weight of the top block, W (weight is the force of gravity acting on a mass; W = mg), acts downward and the resistive force, r acts upward. The net force is mg-r which is set equal to ma (mass times acceleration). We know the acceleration from measurement to be 0.64 mg, so solving algebraically we get that the resistive force is 0.36 mg or 36% of the weight of the top block.

According to Newton’s 3rd law, the forces between two interacting objects are always equal and opposite, so the force the top block is exerting on the bottom block is 36% of its weight. That’s interesting because if the top block were stationary it would exert 100% of its weight on the bottom structure, as it did for many years! How can it be exerting a force less than its weight? Shouldn’t it be exerting a greater force due to its downward velocity, a “dynamic load,” in addition to its weight? Of course it should. If you drop an anvil on your toe it exerts a larger force than if you simply set it on your toe. But observationally we can see that the force of interaction is reduced, not increased! That tells us something is wrong with our assumptions.

What we have here is an underlying structure with a large reserve of strength. The core columns were designed to support 3 times the weight above them, and the perimeter columns were designed to support 5 times the weight above them. Rather than supporting 3+ times the weight, it is failing to support even 36% of the weight. Therefore we can conclude that about 90% of the strength of the supporting columns has been removed. If the support had been completely removed we would have free fall, as in the case of WTC 7. This is not free fall, but this is also not the behavior of a falling block crushing a structure that was otherwise capable of supporting it. To study this analysis in greater detail see my paper in the Journal of 9/11 Studies, “Destruction of the World Trade Center North Tower and Fundamental Physics.”

Graeme MacQueen and Anthony Szamboti explained this phenomenon in perhaps more intuitive terms in a paper entitled, “The Missing Jolt: A Simple Refutation of the NIST-Bazant Collapse Hypothesis.” They asked, where’s the jolt? If you have ever watched an actual pile driver pound pilings into the ground (as when building a pier), a weight is lifted up and allowed to drop repeatedly. As it delivers each blow the reaction force brings the falling weight to rest. It is in fact the transfer of the downward momentum of the driving weight to the pole that creates an impulse, the excess force or dynamic load, that drives the pole into the ground. To deliver that impulse it has to give up momentum, i.e. it has to lose speed. The impact is visible as a jolt. The weight comes to rest, so it must be lifted up to be dropped again and again. But observationally we see that the top section of WTC 1 accelerated downward uniformly, without losing speed, for the entire time the roofline was visible. It did not act like a pile driver because it never gave up its momentum. It accelerated uniformly downward the whole time, not at free fall, but uniformly downward at a slightly lesser rate. Since it never lost its momentum (by never losing its speed) it never applied an excess force to the underlying structure. That is because 90% of the underlying structure had been removed, so it was not there for it to impact. It was falling into the void. Uniform downward acceleration is not free fall, but it has the same implications for WTC 1 as free fall has for WTC 7: explosives were being used to pre-pulverize the building allowing the top section, or what was left of it, to fall with negligible resistance.

For completeness I should mention the south tower. The South Tower, WTC 2, was hit at the level of the 81st floor slab, so the top block in this case was 29 floors tall. By the time the building was about to collapse, firemen had reached the level of the fires and reported that they were relatively small and could be put out with minimal effort. Something else was happening, however, because molten metal could be seen pouring out of the NE corner of the building.

Whether it was actually iron or steel or something else, the color indicates that the temperature was that of molten iron. NIST tried to argue that it was molten aluminum from the plane, but this is a pathetically weak argument. Aluminum melts at a much lower temperature, so even if it glowed it would not be the same color. In fact molten aluminum appears silvery. The significance of all this is that fire produced by burning oil, gas, wood, or most organic materials in open air has a maximum temperature about 1000°F too low to melt iron. This molten iron, or other material at the temperature of molten iron, was caused by something other than burning jet fuel or office furnishings. (Even NIST agrees, by the way, that the jet fuel was burned off in the first few minutes and thereafter the fires were fueled by office furnishings.)

This series of articles is about the way the towers fell, so we are getting a little off track, but molten iron pouring out of the side of the building is a little hard to pass over without mention. We appear to be seeing part of the demolition process in action: pre-weakening with high temperature incendiaries. Shortly after the molten metal appeared, the tower began to fall precipitously.

Unlike WTC 1, the tall top block of WTC 2 immediately began to tip, rotating to the east.

This motion made tracking the downward motion impossible to follow in the same terms as the straight down motion of WTC 1, but the sequence of events that followed defies the pile driver hypothesis. How do you have a pile driver if the driving weight falls off? The motion is a little complicated to describe briefly in detail, but that is essentially what happened. Despite the loss of the driving weight, the lower section of WTC 2 self-destructed all the way to the ground, with explosive ejections ringing the building and running down the building far in advance of the hypothetical collapse front. The explosive demolition process is easy to see. Interpreting this as anything else is very difficult to do.

In Part 6 we return to the central question: what are the larger implications of these observations?

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.