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Free-falling from WTC heights
The towers were 1350 and 1360 feet tall. So let's start by using our trusty free-fall equation to see how long it should take an object to free-fall from the towers' former height.
Distance = 1/2 x Gravity x Time(squared)
2 x Distance = Gravity x Time(squared)
Time(squared) = (2 x Distance) / Gravity
Time(squared) = 2710 / 32 = 84.7
Time = 9.2
So our equation tells us that it will take 9.2 seconds to free-fall to the ground from the towers' former height.
Using our simpler equation, V = GT, we can see that at 9.2 seconds, in order to reach the ground in 9.2 seconds, the free-falling object's velocity must be about 295 ft/sec, which is just over 200 mph.
But that can only occur in a vacuum.
Since the WTC was at sea level, in Earth's atmosphere, you might be able to imagine how much air resistance that represents. (Think about putting your arm out the window of a car moving half that fast!) Most free-falling objects would reach their terminal velocity long before they reached 200 mph. For example, the commonly-accepted terminal velocity of a free-falling human is around 120 mph. The terminal velocity of a free-falling cat is around 60 mph. (source)
Therefore, air resistance alone will make it take longer than 10 seconds for gravity to pull an object to the ground from the towers' former height.
Observations from 9/11
On page 305 of the 9/11 Commission Report, we are told, in the government's "complete and final report" of 9/11, that the South Tower collapsed in 10 seconds. Here is the exact quote: "At 9:58:59, the South Tower collapsed in ten seconds". (That's the government's official number. Videos confirm that it fell unnaturally, if not precisely that, fast. See for yourself: QT Real)
But as we've just determined, that's free-fall time. That's close to the free-fall time in a vacuum, and an exceptionally rapid free-fall time through air.
But the "collapse" proceeded "through" the lower stories of the tower. Those undamaged floors below the impact zone would have offered resistance that is thousands of times greater than air. Recall that those lower stories had successfully supported the mass of the tower for 30 years.
Air can't do that.
Can anyone possibly imagine the supposedly-undamaged lower floors getting out of the way of the upper floors as gracefully and relatively frictionlessly as air would? Can anyone possibly imagine the lower stories slowing any fall of the upper floors less than would, say, a parachute?
It is beyond the scope of the simple, but uncontested, physics in this presentation to tell you how long a collapse should [sic] have taken. Would it have taken a minute? An hour? A day? Forever?
Perhaps. But what is certain, beyond any shadow of a doubt, is that the towers could not have collapsed gravitationally, through intact lower stories, as rapidly as was observed on 9/11.
Not even close!
Because, as you may recall, not only was much energy expended in causing the observed massive high-speed sideways ejections, but virtually all the glass and concrete was pulverized -- actually disintegrated is a much better word. (Nevermind what happened to all the supporting steel core columns...!!!) And the energy requirements to do anything even remotely like that rival the total amount of potential energy that the entire tower had to give. (source) So while gravity is nearly strong enough to cause some things to fall that far, through air, in the observed interval, and while gravity is probably not strong enough to have so thoroughly disintegrated the towers under their own weight, gravity is certainly not strong enough to have done both at once.
Originally posted by ThroatYogurt
Watch the debris...it falls faster than the collapse.
End of story.
Two (2) weeks earlier, NSE had engaged CDI to design a plan to fell the 1,202’-6” tall structure without damage to the helix house 25’ away.
Operating under severe time constraints, CDI was unable to ship U.S.-made linear shaped charges for the project. A CDI representative flew to Buenos Aires where the local explosives supplier had expressed confidence in their ability to melt and pour pentolite explosives in CDI furnished copper sheathing to a density specification provided by CDI.
Inadvertently, and without damage to the helix house, a far more aggressive means of felling large guyed towers was demonstrated. Subsequent analysis of the structural release sequence of the guys has yielded predictable data which CDI intends to use in the felling of large, cable-stayed towers in the future.
Originally posted by IvanZana
Whats gets me is that if the most of the building broke apart and peeled away during the collapse, then what fell all the way throught the core columns to knock em off its welds all the way down to almost the lobby?
Originally posted by Pilgrum
From my observations the collapse got nowhere near free-fall speed and reached a 'terminal' velocity of approx 70mph which is dictated by the resistance it met.
Originally posted by ThroatYogurt
I have seen a video that shows closer to 18 seconds.
On page 305 of the 9/11 Commission Report, we are told, in the government's "complete and final report" of 9/11, that the South Tower collapsed in 10 seconds. Here is the exact quote: "At 9:58:59, the South Tower collapsed in ten seconds".
Regardless, it wasnt a 'natural' collapse, and thats the bottom line.
So I guess the 'squibs' some 30 floors down from the 'collapsing' floors was a floor pancaking on itself? Or perhaps it was an occupant firing a fire-extinguisher the wrong way? No, wait I know, it was a gas mains exploding?
The building was pancaking down from the top and, in the process, blasting air down the stairwell. The wind lifted Komorowski off his feet. “I was taking a staircase at a time,” he says, “It was a combination of me running and getting blown down.” Lim says Komorowski flew over him. Eight seconds later—that’s how long it took the building to come down—Komorowski landed three floors lower, in standing position, buried to his knees in pulverized Sheetrock and cement.