*“Whoever
can be trusted with very little can also be trusted with much, and whoever is dishonest
with very little will also be dishonest with much.”*
Luke 16:10

Some conspiracy theorists are puzzled
about why the WTC towers fell at almost free-fall speed on Sept. 11, 2001. They
suppose that the speed of collapse is evidence that *something* or
*someone* must have destroyed the structural integrity of the undamaged
lower part of each tower.

After all, they reason, *"only the
upper floors of the building were damaged, so why did the lower floors
collapse, and why did they fall so fast?"*

This web page answers those questions, simply enough for even a conspiracy theorist to comprehend (I hope). I do use some simple math and some very basic physics, but even if you don't understand that part you should still be able to comprehend the basic reasons that the towers fell so fast.

What the conspiracy theorists apparently
don't understand is the difference between static and dynamic loading.
("__Static__" means "while at rest," "__dynamic__" means "while
moving.")

If you don't think it can make a difference, consider the effect of a stationary bullet resting on your chest, compared to the effect of a moving bullet striking your chest. The stationary bullet exerts a static load on your chest. A moving bullet exerts a dynamic load.

As a more pertinent example, consider a 110 story building with a roof 1,368 feet high (like the WTC Twin Towers). Each floor is 1368/110 = 12.44 feet high, or approximately 3.8 meters.

Now, suppose that the structural steel on
the 80^{th} floor collapses. (*Note:* I'm using as an example 2 WTC, which
was the building that collapsed first.)

The collapse of the 80^{th} floor drops all
the floors above (which, together, are equivalent to a 30 story building!) onto
the 79^{th} floor, from a height of approximately 12 feet.

Of course, the structure of the lower 79
floors has been holding up the weight of the top 31 floors for many years.
(That's the __static__ load.) So should you expect it to be able to hold
that same weight, dropped on it from a height of 12 feet (the __dynamic__
load)?

The answer is, __absolutely
not__!

Here's why.

First, let's calculate approximately how
fast the upper 30 floors slammed into the 79^{th} floor. (If you slept through
high school physics, you may want to skip ahead to the result.)

* d*=distance,

Solving for

2

Substituting for

Thus

1 meter = 39.37 inches, so

which is

In other words, if you drop something from a height of 12 feet, it will be moving at about 19 miles per hour by the time it reaches the ground. It doesn't matter whether it is a single brick or a 30 story building. After falling 12 feet it will be moving at about 19 mph.

That's about the speed of a collegiate sprinter. (The world record for running the mile is 3:43.13, which is an average speed of 16.134 mph.) If you could sprint that fast and ran into a brick wall the impact might well kill you.

So if the lower 79 floors are strong enough to support a stationary 31 story building, do you think they will be strong enough to support a 31 story building falling at 19 mph?

The answer is emphatically no! But if you
are not convinced, then ask yourself this roughly equivalent question. Suppose
that you can hold up a 50 lb weight with little difficulty. Do you suppose that
you could survive a 50 lb weight falling on you from a height of 12 feet -
i.e., at 19 mph? *(Warning : Do not try
this!)*

To answer that question without killing
someone, I devised the following experiment. First, I found an easily dividable
weight**:** I used my penny jar. Then I made a support for it**:** I used
a piece of notebook paper stretched over a loaf pan, and taped in place. As you
can see, the paper was strong enough to support the
jar**:**

*(click on the photo for a close-up)*

(I was going to determine the limit to the amount of weight it would support, by adding pennies to the jar until the paper tore, but that's all the pennies I had.)

Then I removed the jar from the paper,
and set it aside. I took five pennies from the jar, and taped them together. I
stood on a stepstool, reached as high into the air as I could (about 9 or 10
feet from the floor), and dropped the 5 pennies onto the paper from that
height. As you can see, even though I didn't drop it from a full 12 feet, the
paper still could not withstand the falling pennies**:**

(I took the pennies out of the loaf pan for this photo; that's them next to the lower-right corner of the pan.)

Then I weighed both the five
taped-together pennies (12 grams), and the penny jar full of pennies (1372
grams)**:**

As you can see, 5 taped-together pennies
weigh just 1/114^{th} as much as the penny jar, yet they tore the paper on the
first try. (I didn't try an even smaller stack of pennies.)

You can imagine what would have happened if I'd
dropped the full penny jar on the paper from 10 feet up. If a 12 gram penny
stack broke right though the paper, obviously the paper would hardly have
slowed the 1372 gram jar full of pennies at all... **just as the lower
floors of the WTC towers hardly slowed the fall of the upper
floors.**

That is experimental proof that a stiff (inelastic) structure which can support a given static load may break when less than 1% of that mass is dropped on it from a height of 10 feet. From that fact, it follows that if the full mass which the structure is capable of supporting is dropped on it from a height of 12 feet, the strength of the structure can be expected to slow the fall by less than 1%.

In the case of the WTC towers, there was
a second factor which also slowed the collapse, but not by much. When the top
30 floors of a 110 story building fall 12 feet onto the 79^{th} floor, due to the
collapse of the 80^{th} floor, the mass of the 79^{th} floor is suddenly added to the
mass of the falling structure. The momentum of a 30 story building falling at
19 mph suddenly becomes the momentum of a 31 story building falling at a
slightly smaller velocity. The question is, how much smaller?

* p* = momentum =

Momentum is conserved, so

Solving for

So you can see that the two factors which slowed the fall of the WTC towers were both very small. The strength of the structure below the point of collapse could be expected to slow the rate of collapse by less than 1%, and the accumulation of additional mass by the falling part of the structure due to the the "pancaking" of the lower floors could be expected to slow the rate of collapse by about 3%.

Of course, the above analysis is just
about what happened when the top 31 stories fell onto the 79^{th} floor. To
predict the progression of the entire collapse, you have to repeat the
calculations for each floor. For the next floor, calculate a 32-story building
starting with an initial velocity of about 18.4 mph, and accelerating for
another 12.4 feet to about 27 mph, and then slamming into the 78^{th} floor. Since
kinetic energy is proportional to velocity squared, the falling mass hits the
78^{th} floor with about twice the kinetic energy that the top 31 stories had when
they hit the 79^{th} floor. Obviously, the 78^{th} floor could be expected to slow
the collapse by even less than the 79^{th} floor did, **which is why the building
collapsed at nearly free-fall speed**.

*Dave Burton*

Cary, NC USA

Feb. 21, 2007

P.S. - Here's a video, created by Dave
Thomas of NMSR.org, which illustrates that even a very short drop dramatically
multiplies the force exerted by a falling object, compared to a stationary
object:

Mr. Thomas has also written an excellent article about the physics of the Towers' collapse.

- wtc.nist.gov: National Institute of Standards and
Technology reports & information, including...

- NIST NCSTAR 1, Federal Building and Fire Safety Investigation of the World Trade Center Disaster, Final Report on the Collapse of the World Trade Center Towers (or here)
- NIST NCSTAR 1A, Federal Building and Fire Safety Investigation of the World Trade Center Disaster, Final Report on the Collapse of World Trade Center Building 7 (or here)

- FEMA: World Trade Center
Building Performance Study (or here)
- www.9-11commission.gov: 9-11 Commission
Report
- Scientific American:
Preliminary Opinions of M.I.T. Stuctural Engineers (Oct. 2001)

- Article:
*Engineers blame collapses on fires*

- Article:
*Faulty Fireproofing Is Reviewed as Factor in Trade Center Collapse*

- Cable fire safety info from
www.cablefireresearch.org (look at the photo of what fire can do to
structural steel)
- BBC:
*Q&A: What really happened* - WGBH/Nova:
*Building on Ground Zero* - Popular Mechanics:
*Debunking The 9/11 Myths*(March 2005), and Editor's Notes - Book:
*Debunking 9/11 Myths: Why Conspiracy Theories Can't Stand Up to the Facts*, by The Editors of__Popular Mechanics__ - WGBH/Nova:
*Demolition Woman,*Interview with Stacey Loizeaux (1996) - jod911.com - Journal of Debunking 911 Conspiracy
Theories
- 911myths.com
- 9-11 Myths... Reading between the lies
- debunking911.com - Debunking 911
conspiracy theories
- 429truth.com - a tongue-in-cheek campaign to expose the "truth" about 4-29.
(Refs: [1],
[2]
& [3].)
- Did the U.S. government
plan and execute the 9/11 attacks?
- 9/11: Were Explosives Used?,
by Paul Williams, with interesting Vérinage
videos
- xkcd.com: [1], [2]