Last month, a friend directed me to a video claiming that the September 11th attacks were a masssive conspiracy by the U.S. government. After watching it, I sent her the well-known Popular Mechanics arcticle that deals with most of these claims.
Later that day, she responded by sending me a link to the updated version (linked above) which includes a calculation showing that the towers came down at nearly free fall speed. She said she had checked with an engineering student at Madison, and he had confirmed that if the towers had been pancaking, as the PM article claimed, the would have taken longer to fall.
I then gave something of a lecture on the impossibility of concealing such a conspiracy, and the absurdity of the idea that the whole engineering community would be silent if the PM article was wrong about the pancaking. She replied that people are afraid, with all criticism of Bush being considered unpatriotic. I wasn't all that concerned with the data at that point, but I did say that the pancaking might be accounted for in the ~1s longer it took above free fall (a figure we seemed to agree on).
At the end, I demanded to see the engineering student's calculations and see his data sources. I was ready to get some engineering textbooks and learn, learn the relevant material, and do the equations myself.
After telling her this in a somewhat emotional burst, I paused to think about it for a moment: how fast did the towers really fall? To answer this question, I drew upon a skill I had learned in high school physics: the art of making marks on a video screen.
In my first couple analysis, I used pencil and ruler. My efforts are recreated below using Microsoft's paint program (note that I had to put these into jpg format to get blogger to take them, so they're somewhat fuzzier than my originals, but they should be clear enough when you click for the full versions):
The first picture shows a line at the bottom of the smoke cloud, a line at the burning part of the other tower (to compensate for camera movement) and a line across one tower to establish a scale. This scale would be 91 pixels:64 meters (at least it's 91 pixels on my computer). The 64 meter figure for the width of the tower may be found here:
In the second picture, we see that the camera has followed the fall 131 pixels down, while the bottom of the dust cloud is 10 pixels higher on the screen. This means the tower has fallen 121 pixels, or ~85.1m.
As best I can tell, the number after the colon on the timer is 30ths of a second. Put in decimal form, we are 5.73s in to the fall. From all this, we have
85.1m = 1/2 * a * (5.73s)^2
a = 2 * 85.1m / [(5.73s)^2] = 5.18m/s^2
(5.18m/s^2) / (9.8m/s^2) * 100% = 52.8% of freefall acceleration
When I attacked this with ruler and pencil, I got answers from 53% to 56%. I suspect that this is perfectly consistent with pancaking, though I only sent these screen shots to my friend today, and she hasn't found out from the engineer how long pancaking should have taken. This is something of an ongoing story, I may update this if the situation warrants it.