Why did the collapse of WTC take only 9.5 seconds?

Question

An object no matter the mass is which fall from the same height will take the same time to reach the ground (Galileo's Pisa Experiment).

If the height of WTC is 526.3 meters and the gravity constant is 9.8 m/s^2 then the time needed by the roof top to reach the ground is given by:

h=1/2*g*t^2
t=sqrt(2*h/g)
=sqrt(2*526.3/9.8)
=sqrt(107.4081633)
=10.36379097 second

but why in the video recording it took only 9.5 seconds. It is so odd isn't it?

#Both of the towers burned for a lot longer than 9 seconds!

I meant the top floor reached the ground by only 9.5 seconds. I did not say the burning process, I said the duration the fall take, as a point of reference we can take the position of the roof top.

#(By the way, your measurements don't take into account that the towers collapsed from the middle, not from the bottom. Suspiciously, the collapse began precisely where the planes hit, not from the bottom as they would have if they had been demolished by explosives.

No, look here: Why do everything falls to earth? Because the earth has gravity force, the force is 9.8 m/s^2. This is a simple senior high physics question: if there is no external force, then a solid structure, no matter the mass, will fall with the speed and duration given by the Newton's Laws. So the top of WTC is 523.6 meters above the ground.

It cannot reach the ground faster than 10.36 seconds as I calculated. Except there is an external force from the foundat

#....and by the way....Galileo's Pisa Experiment was disproven in 1998.

Yes, it is not 100% correct, regarding the effect of relativity. But it is confirmed by Newton's Law. And in mechanical / civil engineering problems the error of Newton's Law is very very very very very very very small. Even in Maxwell's Electromagnetics Theory, the effect of relativity is "relatively" neglectable

We can say Galileo's experiment is right for every rigid objects (except aerodynamics) if the speed < 0.01 C (less than 3000000 m/s)

#So you are trying to say that Bush and the CIA changed the laws of physics in order to frame Osama? Wow, Bush is God now.

No, I'm just curious really... No one need to be God or breaking the laws of mechanics to perform that phenomenon. We only need an external force from the foundation of the building to speed up the collapse process.

#There is somekind of compression process in the materials so that the force from the top accumulate so the process can be faster than your simple prediction.

I don't know for sure, but if you got an A+ in 10th or 11th grade physics, you should know that gravity is a conservative equation. It means if you fall you cannot speed up your falling time. If a building collapse, nothing can increase the force of gravity so that the falling time faster. The properties of conservative fields are given in college Vector Calculus, you should read Kreyszig's books, Purcell-Varberg books, or other related topics.

The only way can is an external force. In the case your falling from building, the only way to speed up the time you reach the ground is: you need an external force from the ground (someone pulling you)


#A force at the "foundation" would produce the same free fall speed for the WTC as a force at the top. Gravity is gravity.

Simple if you're falling you are pulled by the earth with 9.8

#A force at the "foundation" would produce the same free fall speed for the WTC as a force at the top. Gravity is gravity.

Simple if you're falling you are pulled by the earth with 9.8 m/s^2 force. The way to modify this conditions: 1. A positive magnitude force applied to you (someone pulls you up or you bring a chute so that the wind gives you a postive magnitude force) 2. A negative magnitude force applied to you (somone pulls you down with a rope or the wind blows you down.

Why only an external force from the ground: Because the ground is the only thing that doesn't move. If you're playing chute with your friend, there's no way you can pull her/him because you are affected by a conservative force (in this case gravity) too.

This is not about the absurd conspiracy theory. This is just an odd phenomena keep bugging in my head as I heard the news about the freedom tower that they will build in ground-0.

I just want to see how it destroyed that time, and bingo a single question just shot my head - isn't that way too fast? As I calculate it yes it is too fast. I'll seek a quote from a bonafide source that the duration was 9.5 seconds; by the way I got a lecture to do

#in fact, THE MAIN FORCE used in a controlled demolition IS THE BUILDING'S WEIGHT ITSELF.

YES IT IS; at last someone got my point....
And that is THE POINT OF MY QUESTION... About the 9.5 seconds, you can find the video from various source like kazaa, CBS, photobucket, google search it is the first tower's collapse.

I'll show you how the 9.5 seconds come from later. But all the source give the same time. I'll show you later, I got a lecture.
I don't know whether they speed up the duration but I've measured from different sources.

About 9.5 seconds:

"the building to fall in about 8.3 seconds, whereas the observed roof-fall" quoted from:
www.journalof911studies.com/articles/W7Kuttler.pdf

"If the entire building is to collapse in 9.5 seconds,"
www.gnn.tv/threads/6031/Former_Bush_Team_Member_Says_WTC_Collapse_Likely_A_Controlled_Demolition

You can search in Google

This is a physics discussion not a dirty words contest.

I got a lecture to go... We'll discuss later

O yeah, 1 more thing: I'm a Physics Nerd...

I have read Young's Physics book and Haliday-Resnick as well. I got straight A+ in Physics and Maths: Algebra & Geometry in senior high.

#it lands on top of the mountain of debris from the 110 floors below it.

Yes, the debris height is less than 4m, so it only reduce the time only less than 0.01second yet it's still > 10.3 seconds

#Also, even if you buy into the idea that all the floors were blown out simultaneously, it's still going to take the roof that 10.36 seconds to fall to the ground.

YES YOU'RE RIGHT, THAT IS THE POINT. What "external force" does take it faster than gravity"
I am not speculating about demolition or bombing, look here I'm asking why it faster than gravity. If I had made an assumption, I won't ask it in Yahoo Answer.

And yet, I DO NOT SUPPORT THE CONSPIRACY THEORY.

Answer 1

The answer is really quite simple and proven from your calculations which are indeed correct. You say that it should have taken about 10.3 sec. for an object to fall about 530 m at the acceleration of gravity. I find this to be perfectly reasonable. If the building completely collapses in 9.5 sec., requiring an acceleration greater than that of gravity alone, the video is clearly altered. There is no way in a controlled demolition to achieve a greater downward force than the weight of the building (force due to gravity), in fact, the main force used in a controlled demolition is the building's weight.

I see no plausible explanation of how one can physically achieve an acceleration greater than that of gravity in this situation; therefore, I must conclude that the video you are watching is altered in some way.

Answer 2

I'm not sure how they arrive at that 9.5 second figure but I think there are some erroneous assumptions in there. It's not really possible to determine from the video when the top floor reaches bottom, because, by that time everything is lost in a cloud of dust. In any case, the roof doesn't really reach the ground - it lands on top of the mountain of debris from the 110 floors below it.

I suspect they may have timed from the beginning of the collapse at the top to the first major seismic shock. However, that is not the roof hitting the ground. It's the point where the falling upper section of the building (the collapse doesn't start at the roof) meets solid resistance from the pile of debris it's been driving ahead of it. The seismic records show the collapse going on for another several - like 10 or 15 - seconds after this.

Also, even if you buy into the idea that all the floors were blown out simultaneously, it's still going to take the roof that 10.36 seconds to fall to the ground. The only way to speed up the fall would be to have a massive shock wave above the building pushing it down. So all the conspiracy promoters achieve in this case is a convincing demonstration that they have misinterpreted the evidence and misunderstood the laws of physics.

Answer 3

So you are trying to say that Bush and the CIA changed the laws of physics in order to frame Osama? Wow, Bush is God now.

You should like the article at the link below, then:

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Assuming that you are correct, then even if there was an “external force” from the “foundation” (like an explosion deliberately set off by the US government), the WTC would still have to fall down at the speed set by Newton’s Laws. Period.

Even if the CIA & the Bush Administration had brought down the WTC on purpose (which you seem to be suggesting), they still would have had to obey the same laws of physics that you see everywhere else. Are you suggesting that the CIA has a tractor beam or something that scientists don’t know about?

What is your point? That Bush is God, and that he can change the laws of physics? Don't you see the illogic of what you are suggesting?

===edit2===

A force at the "foundation" would produce the same free fall speed for the WTC as a force at the top. Gravity is gravity.

Answer 4

Because the video is deceptive.

Internal collapses are not distinguished in the view from the distance required to view the tower in it's entirety, the angle of the camera view is an unknown variable, and there is no way to actually time when the "top" hit the actual ground. Also, the top debris landed on the collapsed material which piled on top of the actual "ground zero" to shorten the distance which material had to fall.

Just as an airplane traveling at over 600MPH at 35,000 feet looks like it is traveling at a snails pace.

.....and by the way....Galileo's Pisa Experiment was disproven in 1998.

Answer 5

Physics was never my best subject because I was "taught" by some fatass football coach whose only instruction was to "study from the book". I kid you not, he didn't try to teach us ANYTHING. :S

In the video you saw, the last bit of the tower itself was probably hidden behind the huge dust cloud billowing up beneath it. You only SAW 9.5 seconds of falling, when it was probably taking the whole 10.4 seconds, assuming your calculations are correct.

Answer 6

be careful with questions regarding the world trade center. you could be opening Pandora's box with this question. many people on this site do not take kindly to the inclination of any conspiracy theory involving 9/11. some simply don't like dealing with the issue at all. don't be surprised if you receive some very hostile answers.

but your question IS an intriguing one.

Answer 7

They had to blow the floors out a little faster than free fall speed, otherwise the intact lower structure would have caused the upper floors to topple off to one side.

Answer 8

Because it was all a conspiracy led by the hands of the Bush admin. Well that's what you wanted to hear right? idiot.

Answer 9

yes