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Originally Posted by COGENT
But to answer the question what I think "shoud've" happened.
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This is the root of many problems, and again is a well known cognitive bias. We but FAR too much trust in our mental models of the world around us. In the same way that Newton's theory of gravity only works with specific ranges of reality, our mental models of reality are informed by a limited subset of reality.
This is because evolved and grew up dealing with situations where the material strength far exceeds the stresses placed on that material by gravity and mass (for example). Our experiences do not equip our common sense to deal with extreme situations. This is easily seen by the hilarious videos on youtube where people try to move or lift heavy things.
This cannot be overstated, and this is why math is used to build buildings, not "I think this will work" guessing, and this is why math MUST be used to analyze the collapse of the structures as well.
Quote:
Originally Posted by COGENT
To start, Neither of the planes hit straight on and they hit near the tops of the buildings. I would think that if anything, the top of the building would tip to one side either breaking off or causing the rest of the building to tilt to one side while collapsing. I guess my idea of this comes from something as simple as jenga or a tree being chopped. take out one side of it, it falls to the side. Although I do understand that the engineering of two of the largest buildings in the world and jenga are very different.
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VASTLY different. This is exactly what I mean, your experience is informed by small things, and you extrapolate (as any reasonable person would) out based on that. The problem is things don't extrapolate like that. Acceleration due to gravity is a square relationship. Mass and volume have a cube relationship. These aren't things we model very well mentally.
So in the case of the tops of the buildings, even though the planes were quite big and going quite fast, the actual energy involved still wasn't that big compared to the sheer inertia of the building. In order for something to tilt you have to overcome the inertia of the part that's tilting, and you have to have something rigid to support one side of the tilt as one side rotates and not the other.. the building was not designed so that the supports on one side of the building could fully support the weight of the above structure while the other side were 100% compromised (which is what would be required to have the top tilt). When the mass is so huge and the forces due to acceleration due to gravity dwarf all the other forces, straight down is pretty much the only option.
Quote:
Originally Posted by COGENT
Furthermore. How does a fire on the 93rd floor cause building to completely collapse in on itself within an hour and thirteen minutes? I would think that this would require serious structural problems throughout the whole building below the 93rd floor. You would think if the buildings were desinged/able to withstand a plane flying into it at 500 mph it could handle a fire on 6 of it's floors and not weaken it's structural strength throughout the building.
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The structural strength of the lower 93rd floors is pretty much irrelevant in this case.
The reason the whole building collapsed is because one single floor collapsed, that's it. Again without math it's hard to understand because we don't usually deal with the kinds of forces involved in every day life, but this is one case where I can use an analogy that we do deal with.
Take a 25lb bag of sand. If I'm holding it in my arms, I'm stopping it falling right? That load on my arms is its static load. Now if I pass that to you and you take it, nothing bad happens because we do it in a way to make sure the dynamic load is minimized. But if I raise that bag 10 feet in the air and drop it
into your arms, are you going to try and catch it? Of course not, because you know the dynamic load is going to be enough to injure you.
So same thing here. We've got 14 floors of building putting a static load on the next floor down. The fires burn, steel weakens, the floor trusses expand and sag, and eventually enough structural integrity for one floor erodes that the top 14 floors drop. Once this happens it's all over, because remember the sandbag; the forces involved due to acceleration of that mass due to gravity FAR exceed the structural integrity of the next floor fully built.
I've worked through the math on this one myself and even taking into account the support of partially compromised steel supports, the dynamic load on the floor below as a result of the 14 floors above falling just one floor is more than 30 times the static load.
So unless the 92nd floor can support 30 times the force it normally has to deal with, it will collapse. It does, and its mass is added to the falling mass, the mass accelerates further through the 92nd floor, and the 91st floor has to deal with even MORE force, which it can't, etc etc..
Quote:
Originally Posted by COGENT
Exactly.... magnets.... what's their deal anyways?
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Feynman said this: "One part of the force between moving charges we call the magnetic force. It is really one aspect of an electrical effect."
Magnetism is what you get when you combine electricity and special relativity. Lorenz transformations across inertial frames of reference and all that stuff I don't understand nearly as well as I wished.
