# Free Fall — Part 2

Apr 14, 2018 · 7 min read

Part 1 2 3 4 5 6

If you haven’t read Part 1 of this series, you should really start there.

In Part 1 we saw that the free fall of World Trade Center Building 7 (WTC 7) on 9/11/01 was an observational reality, independent of any politics, how or why questions, or any theories, conspiratorial or otherwise. It was an objectively measurable fact. The central question asked in Part 1, which has not yet been answered, was, “What does it take for a tall building to collapse downward through its own structure in absolute free fall?” Let us approach this question by looking at the physical implications of free fall.

So what does free fall imply?

We can state with certainty that during free fall the falling section of the building could not have been what was destroying the structure underneath it. The clearest way to understand this is to consider the energy involved. When an object is elevated it has potential energy. In free fall, the potential energy is converted entirely into kinetic energy with no energy left over to do anything else.

Let’s unwrap that statement.

Think of a mouse trap. When you load a mouse trap you have to apply a force to the spring as you move the spring into position and lock it. When we apply a force to move an object we say we have done work on it. Another way to say this is we have given energy to it. Energy is what you transfer to an object when you do work on it. Once loaded, the trap has the potential to do the same amount of work on something else when the trap is released, so we say the loaded trap has potential energy — spring potential energy in this case. Doing work on an object and transferring energy to the object are two ways of talking about the same thing.

Lifting a bowling ball in a gravitational field is a lot like loading a mouse trap, but in this case we do work against gravity instead of a spring. The lifted bowling ball has potential energy equal to the work done in lifting it: gravitational potential energy, in this case. We call it potential energy because, as in the case of the mouse trap, the energy can be retrieved and changed into other forms when the ball is allowed to fall. You wouldn’t want to drop it on your foot because it will dump its potential energy into your foot and perhaps break some bones. A more purposeful use of the potential energy of a raised bowling ball is to allow it to swing back, then swing down in a circular arc, gaining speed as it goes down, and release it as it is moving horizontally at floor level. This converts the potential energy into kinetic energy, energy of motion, which we see as the ball speeding down the lane.

When the ball hits the pins it transfers some of its energy to the pins, causing them to be knocked around. Some of the ball’s energy also goes into producing sound, heat, and deformation, as dents are formed in the pins. The energy transferred into these other forms is deducted from the kinetic energy of the ball, causing it to slow down. Throughout this process, from lifting the ball to swinging it to the floor, to hitting the pins, the total energy is conserved. It all adds up to the original energy at the beginning of the process. The energy is simply converted from one form to another as it is passed around to the various objects in the system. Calculating with energy amounts to bookkeeping.

A tall building has potential energy given to it as it was built by cranes that lifted the heavy components into place. As in the case of a loaded mouse trap, that energy remains, hidden and seemingly passive, as the building is held in place for years. That stored energy is released, however, if the building is demolished. In a conventional demolition, support is removed low in the building, allowing the top section to fall. As the building falls, its potential energy is transferred into other forms. If it does not interact with anything along the way, all of the potential energy is converted into kinetic energy of the downward moving building. This is a description of free fall. If the falling section crushes or otherwise interacts with the underlying structure, the energy is shared among various forms: energy of deformation of the structure, kinetic energy carried off by objects being thrown around, and heat generated by these processes. Any energy transferred to these other processes is deducted from the kinetic energy of the falling top section of the building, causing it to slow as it falls. The only way free fall can be maintained is for none of the energy to be used for other purposes. In free fall, the potential energy is converted to kinetic energy of the falling mass, and nothing else.

In the case of WTC 7 the fact of an extended period of free fall has been established by direct observation and measurement. See Part 1 of this series. We can conclude that all of the potential energy was being converted into kinetic energy with nothing left over to do anything else. Only after the 2.5 second mark, when the building ceased to be in free fall, does the falling section of the building start to interact with the structure, and transfer its energy into other forms. In a natural collapse there is no free fall. It is the descending building itself that demolishes the underlying structure as it collapses. In a commercial demolition there may be a short period of free fall, or near free fall, when the support is initially removed, but for the sake of economy the top section of the building is allowed to contribute its enormous potential energy into the demolition process. The initial free fall, if any, had to be induced from some external energy source, usually explosives. In the case of WTC 7 it is clear that there was enough destruction due to an external source of energy to clear the way for eight stories of free fall. The free falling building did not destroy those eight stories. It was the clearing away of those eight stories by other means that allowed the free fall to occur.

We are now able to answer our original central question: “What does it take for a tall building to collapse downward through its own structure at absolute free fall?” The answer is the underlying structure has to be removed, and the energy needed to accomplish this has to be supplied from some external source. The free falling section of the building does not contribute to this process.

What could the external source of energy be? In simple demolitions energy is provided by a wrecking ball that attacks the structure piecemeal. Like a wrecking ball the Twin Towers were hit by airplanes, but like the wrecking ball these impacts caused only localized damage which the towers survived for an hour or more. WTC 7 was not hit by an airplane. Earthquakes can cause buildings to fail, but there were no earthquakes in New York City that day. Verinage demolitions, innovated in France, are accomplished by simultaneously buckling all of the support columns on the middle floors using hydraulics or sometimes cables. Fire can cause wooden structures to collapse, but fire tends to eat away at a structure, causing a series of local failures that may culminate in a global collapse. Fire, on the other hand, has never caused the complete collapse of any steel frame high rise, even after burning for many hours. The only way, apart from Verinage demolitions, buildings have ever been brought straight down with sudden onset, and with an initial period of free fall, is by using explosives to suddenly and simultaneously remove all column support. All of these mechanisms, apart from earthquake or fire, would seem to require rather major outside intervention.

I ended Part 1 by pointing out that I was not engaging in any kind of conspiracy theory, because I did not put forth any theory whatsoever, conspiratorial or otherwise. The same remains true here. I have simply provided the background physics sufficient to understand the nature of the problem.

After a great deal of pressure and a long delay, the Bush administration commissioned an analysis of the collapses of the Twin Towers, completed in 2005, and WTC 7, completed at the very end of the Bush presidency in November 2008, by NIST, the National Institute of Standards and Technology. Many people assume that NIST is an independent scientific agency, but in fact NIST is a government agency under the Commerce Department, which is in the executive branch of the government. Keeping that in mind, we look at the analysis provided by NIST in Part 3.

Part 1 2 3 4 5 6

Written by

## David Chandler

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

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