It should take a brick from the top of the WTC1 about 9.2 seconds (in the vacuum of space). Adding for air resistance an a terminal velocity of about 150 km/h I'd say 13 seconds. One tower fell in about 16 seconds and the other in about 18 or 20 seconds if not mistaken. So that's roughly 70% I guess.
JM,
Do you know what REALLY cheeses me off?
When someone completely screws up the analysis ...
... and still comes up with the right answer.
For an object dropped from a zero initial vertical velocity from some height (h):
d = 0.5 a t
2
a = 2 d / ( t)
2
Now the question is "what is the correct value for d?"
I'll give you a hint: a semi truck is heading for a wall. Do you register the impact as happening when the front bumper hits the wall, or when the rear bumper finally gets there?
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Background:
The Truthers initially were aghast that debris hit the ground at, essentially free fall acceleration. Well, a bunch of material DID fall at this acceleration: the material that fell outside the footprint of the building and WAS, therefore, in free fall.
Big "Duh".
When people pointed out that there are images of external panes at about the 20th floor height, while the collapse zone is around the 60th floor, it finally occurred to SOME truthers that the acceleration of the material thrown clear of the building was irrelevant to what was happening within the footprint of the building.
Other truthers have never caught on to that concept.
I'll assume that you're in the first group.
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Regardless, the initial accelerations of the upper blocks of the towers (first 15 stories, until the block disappeared into the cloud of debris) were accurately measured at about 0.7g & 0.75g for the two buildings.
Then, as the buildings accelerated, resistance forces built up until they achieved a fairly constant speed. But this is extremely difficult to measure, because they descended into obscuring clouds of debris.
So, if you do a force diagram, you arrive at the 1st order approximation that the lower portion of the building applied an AVERAGE force to the upper blocks over this initial 15 story drop of about .3 W, where W is the weight of the upper block.
The upper blocks weighed about 50,000 tons (IIRC).
That means that the lower portion of the buildings applied a resisting force, during the initial collapse, of around 15,000 tons.
This is not an "insignificant" amount of force.
This is not "almost no resistance".
