I think that site has it's own agenda (Daaah), and if you have seen the latest Discovery channel piece, I think you will see it is not impossible. And quite a bit of forensics was done, and a significant portion of the debris was kept...
The mass of the upper floors is only slightly slowed by the relatively insignificant mass of the individual floors below as they impacted. Remember, the pancake does not start at the top, it starts at the point of impact, or possibly below. Also, both the above and below sections can pancake at the same time.
And there is no way the top floor ever reaches ground level, so the whole drop an object vs. the top of the building thing is silly.
A number of good scientific web sites on the collapse... Here is one relevant quote.
Quote:
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Originally Posted by http://www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.html
As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell. The floor below (with its 1,300 t design capacity) could not support the roughly 45,000t of ten floors (or more) above crashing down on these angle clips.
This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h
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