Billy of Hayward, California wrote:
I believe that NIST says that the fire was so hot that it caused the building's trusses to expand, causing the beams to bow out and effecting a perfect pancaking effect. The momentum vector would then have to be perpendicular to the ground and right down the center of the buildings. This would not be possible even if the expansion of the trusses was linear (and only in college at the BS level do we get to play with "linear"). If it could happen, if each truss could expand at exactly the same rate to bend the beams at exactly the same rate, you might be able to prove this case for the North Tower. Each collision of one floor onto another would have a cumulative effect on the total final vector (in the real world it still would not fall straight onto the next level because materials have different impurities at different places, breaks would happen, different people weld differently, etc). Just like the theory of how the planets were formed in the early solar system - each successive collision (with different vectors) of matter helped to create the final angular momentum vector, known as the axis of rotation.

But the South Tower started with an initial angle of the top floors, so how was this angle kept throughout the fall when all it wants to do is tip over? Each collapse would need to cause a counteracting angular momentum vector. Or, all supports would need to be canceled at once, causing a near free fall (Hmmm, I can't choose between these, I'm really fond of the truth but I'm lazy and tired of independent thinking.) Oh well, I guess I will go ponder the immaculate conception over some tea.

Former Aerospace Controls Engineer
NASA - Lockheed Martin