Merged 9/11 CT subforum General Discussion Thread

[BigAl]

9/11 ground zero wtc

One of the pieces of steel from WTC-9/11 has been erected near Vesey & Church a few yards from the NE corner of the WTC construction fence. It's an I-beam about 12x14 inches with 3/4 thick sections.

Does anyone know what part of WTC this beam might be from?

In in one of The Jones-is-an-idiot threads people were asking about the paint/rust/therm*te/whatever that might be on a beam. Here's your chance to touch the holy grail. You can get right up to it via a gate in the fence while it stays open. I'd hate to see graffiti on this.

Right now it appears to me to be course rust, but I'm not a ther*te expert
and it is a random beam, one of thousands.

 

[bill smith]

Suppose the plane was stationary in the sky and you flew the building at it at neaarly 600mph ? Are the forces the same at impact and would the plane still go through the outer columns,and hit the core ? How would the force that caused the building to bend be represented in this case ? What would we see ?

To have a go at answering some of my own question;-

It looks to me like the building would have much more mass and momentum than the plane would have mass and inertia. But the forces at the point of impact would be identical to the original event and the local damages should be the same if the plane can really go through the outside columns as we saw on the day.....in other words the core should still hit the plane. Am I right ?....But what happens then ?

 

[bill smith]

To have a go at answering some of my own question;-

It looks to me like the building would have much more mass and momentum than the plane would have mass and inertia. But the forces at the point of impact would be identical to the original event and the local damages should be the same if the plane can really go through the outside columns as we saw on the day.....in other words the core should still hit the plane. Am I right ?....But what happens then ?

This is a contribution to his conversation by tfk on another thread. Jeez...this is like getting blood out of a stone..

Originally Posted by tomk
When two things collide, which one is moving and which one is stationary does NOT change anything. This is a cornerstone of both traditional mechanics and relativity.

But you have to get the "constraints" to be the same for the two cases. That means "how rigid are the pieces held in place" has to be identical. And, in general, things that are moving tend to be light and softly constrained, while things that fixed into the ground tend to be heavy and firmly fixed.

The plane cannot be firmly fixed to the ground tho. It has to be able to "accelerate" backwards as a result of the collision just like the moving plane was able to "decelerate".

...

If you get the constraints set up right, you'll get exactly the same results if you throw the building into the plane at 500 mph as you did by throwing the plane into the building at that speed.

 

[JoeyDonuts]

If you get the constraints set up right, you'll get exactly the same results if you throw the building into the plane at 500 mph as you did by throwing the plane into the building at that speed.

Conservation of momentum? The motive force required to accelerate an object as large as WTC to 500 mph being greater than what's required for...



....ah, never mind.

 

[Dave Rogers]

It looks to me like the building would have much more mass and momentum than the plane would have mass and inertia. But the forces at the point of impact would be identical to the original event and the local damages should be the same if the plane can really go through the outside columns as we saw on the day.....in other words the core should still hit the plane. Am I right ?....But what happens then ?

Exactly what happened on the day, provided that, in your example, the earth and the atmosphere and all other objects stationary with respect to them are also travelling at 500mph in the same direction as the tower. In fact, what happens is not just the same as what happened on 9/11; it is what happened from 9/11, described from the point of view of a hypothetical observer travelling alongside the airliner at the same speed.

In basic physics terms, all inertial frames of reference are equivalent; an inertial frame of reference is one experiencing no acceleration i.e. travelling at constant velocity. If you transform all the events of 9/11 into a co-ordinate system in which a moving building strikes a stationary airliner, all you're doing is adding an additional term to one co-ordinate of every object present, which is linearly proportional to time. This term will cancel out in the maths, leaving no overall difference in the result.

Dave

 

[bill smith]

Exactly what happened on the day, provided that, in your example, the earth and the atmosphere and all other objects stationary with respect to them are also travelling at 500mph in the same direction as the tower. In fact, what happens is not just the same as what happened on 9/11; it is what happened from 9/11, described from the point of view of a hypothetical observer travelling alongside the airliner at the same speed.

In basic physics terms, all inertial frames of reference are equivalent; an inertial frame of reference is one experiencing no acceleration i.e. travelling at constant velocity. If you transform all the events of 9/11 into a co-ordinate system in which a moving building strikes a stationary airliner, all you're doing is adding an additional term to one co-ordinate of every object present, which is linearly proportional to time. This term will cancel out in the maths, leaving no overall difference in the result.

Dave

\Thanks for that Dave. But leaving it to one side for a minute how can we make a situation that accurately reflects the picture I am trying to set up ? One where all the forces are accurately rreversed ?

Assume that the plane is fixed as solidly in in the sky as the building was in the ground. So the flying Tower hits the fixed plane, the plane goes through the outer columns and the core hits the plane . In the real event the building bends back 27 inches. What happens in this situation ?

 

[Dave Rogers]

Assume that the plane is fixed as solidly in in the sky as the building was in the ground. So the flying Tower hits the fixed plane, the plane goes through the outer columns and the core hits the plane . In the real event the building bends back 27 inches. What happens in this situation ?

If you really mean that the plane is fixed in position, then there must be some force applied to it as the tower strikes to maintain it solidly in position; since this force has no analogue in the real events of 9/11, something different would happen. If, on the other hand, the plane were intially stationary but no force were present to maintain it in position, and the tower hit it, the tower would experience an off-centre decelerative force which would cause it to slow and rotate. In the absence of anything restraining the rotational motion of the tower it would continue to rotate. Meanwhile, the debris from the airplane would be accelerated, with the parts that remained in the tower being accelerated up to the speed of the tower and the remaining parts being accelerated less, emerging from the back of the tower and continuing in the same direction at a lower speed. Again, the absence of the restraining force from the connection to the ground is a significant difference from the real events.

In fact on 9/11 the tower was firmly fixed to the ground at its base, so, although it experienced the initial rotational impulse from the off-centre impact, this was arrested by elastic deformation of the structure. This elastic deformation was then transformed into movement in the opposite direction, which then was converted into an elastic deformation in the opposite direction, with some energy loss in the process. Therefore, the building then bent forward somewhat less than 27 inches. It then rebounded a second time, and so on in a decaying oscillation as the energy of the initial impact was dissipated.

Again, if the events were described in terms of the tower and the ground moving at 500mph (or whatever the final speed of the airplane) and striking a stationary airplane, the results would be exactly the same (with the exception that, instead of coming to rest, everything would end up moving at 500mph).

Dave

 

[bill smith]

If you really mean that the plane is fixed in position, then there must be some force applied to it as the tower strikes to maintain it solidly in position; since this force has no analogue in the real events of 9/11, something different would happen. If, on the other hand, the plane were intially stationary but no force were present to maintain it in position, and the tower hit it, the tower would experience an off-centre decelerative force which would cause it to slow and rotate. In the absence of anything restraining the rotational motion of the tower it would continue to rotate. Meanwhile, the debris from the airplane would be accelerated, with the parts that remained in the tower being accelerated up to the speed of the tower and the remaining parts being accelerated less, emerging from the back of the tower and continuing in the same direction at a lower speed. Again, the absence of the restraining force from the connection to the ground is a significant difference from the real events.

In fact on 9/11 the tower was firmly fixed to the ground at its base, so, although it experienced the initial rotational impulse from the off-centre impact, this was arrested by elastic deformation of the structure. This elastic deformation was then transformed into movement in the opposite direction, which then was converted into an elastic deformation in the opposite direction, with some energy loss in the process. Therefore, the building then bent forward somewhat less than 27 inches. It then rebounded a second time, and so on in a decaying oscillation as the energy of the initial impact was dissipated.

Again, if the events were described in terms of the tower and the ground moving at 500mph (or whatever the final speed of the airplane) and striking a stationary airplane, the results would be exactly the same (with the exception that, instead of coming to rest, everything would end up moving at 500mph).

Dave

A nice clear explanation. I'll come back on it later when I've got more time. I'll have to take it all in first. Thanks.

 

[bill smith]

If you really mean that the plane is fixed in position, then there must be some force applied to it as the tower strikes to maintain it solidly in position; since this force has no analogue in the real events of 9/11, something different would happen. If, on the other hand, the plane were intially stationary but no force were present to maintain it in position, and the tower hit it, the tower would experience an off-centre decelerative force which would cause it to slow and rotate. In the absence of anything restraining the rotational motion of the tower it would continue to rotate. Meanwhile, the debris from the airplane would be accelerated, with the parts that remained in the tower being accelerated up to the speed of the tower and the remaining parts being accelerated less, emerging from the back of the tower and continuing in the same direction at a lower speed. Again, the absence of the restraining force from the connection to the ground is a significant difference from the real events.

In fact on 9/11 the tower was firmly fixed to the ground at its base, so, although it experienced the initial rotational impulse from the off-centre impact, this was arrested by elastic deformation of the structure. This elastic deformation was then transformed into movement in the opposite direction, which then was converted into an elastic deformation in the opposite direction, with some energy loss in the process. Therefore, the building then bent forward somewhat less than 27 inches. It then rebounded a second time, and so on in a decaying oscillation as the energy of the initial impact was dissipated.

Again, if the events were described in terms of the tower and the ground moving at 500mph (or whatever the final speed of the airplane) and striking a stationary airplane, the results would be exactly the same (with the exception that, instead of coming to rest, everything would end up moving at 500mph).

Dave

Would it not be true to say for the purposes of this hypothetucal that if the building is assumed to take on the velocity of the plane that the plane must be assumed tp take on the inertia of the building ?

 

[BigAl]

Would it not be true to say for the purposes of this hypothetucal that if the building is assumed to take on the velocity of the plane that the plane must be assumed tp take on the immobility of the building ?

Can people get out of High School without a science course?

 

[TjW]

Would it not be true to say for the purposes of this hypothetucal that if the building is assumed to take on the velocity of the plane that the plane must be assumed tp take on the inertia of the building ?

No.

 

[bill smith]

No.

Did you cut your signature short ?

 

[Dave Rogers]

Would it not be true to say for the purposes of this hypothetucal that if the building is assumed to take on the velocity of the plane that the plane must be assumed tp take on the inertia of the building ?

Let me get back to you on that one. It's wrong, but I'd like to take some time to work up a clear explanation of how and why it's wrong.

Dave

 

[Dave Rogers]

Would it not be true to say for the purposes of this hypothetucal that if the building is assumed to take on the velocity of the plane that the plane must be assumed tp take on the inertia of the building ?

Right, let's try and pick this one apart. I think the problem may be that you haven't got the experience of stating physics concepts clearly, so I'm not entirely certain what you mean. However: In whatever frame of reference you view the events, there is a transfer of momentum between the building and the airliner, brought about by the force between them that deforms and fractures their respective structures. This force can only oppose their relative motion, so at the point where there is no relative motion there can be no force; in other words, once the building and the aircraft are moving at the same speed, they will continue moving at the same speed. Because the mass of the building is so much greater than the mass of the airliner, that final speed is very close to the initial speed of the building.

It's all most easily expressed in terms of conservation of momentum, and conservation of momentum works in any inertial frame of reference. If we view the building as being stationary, its initial velocity is zero, and its final velocity is the velocity of the airliner, multiplied by the mass of the airliner, divided by the sum of the two masses; since the mass of the building is about 1800 times that of the airliner, the final speed of the building + airliner is very much smaller than the initial speed of the airliner - about a quarter of a mile per hour. Still, quite high for something that's fixed to the ground. If, on the other hand, we view a moving building as hitting a stationary airliner, then the final velocity of building + airliner is the velocity of the building, multiplied by the mass of the building, divided by the sum of the two masses; since the airliner is so much smaller than the building, that's very nearly the same as the initial velocity. In fact, it's about a quarter of a mile an hour slower.

So, if the plane is moving and the building isn't, the plane only moves the building relatively slowly; and if the building is moving and the plane isn't, the plane only slows down the building a tiny amount. All this simply follows from the ratio of the masses.

Dave

 

[bill smith]

Right, let's try and pick this one apart. I think the problem may be that you haven't got the experience of stating physics concepts clearly, so I'm not entirely certain what you mean. However: In whatever frame of reference you view the events, there is a transfer of momentum between the building and the airliner, brought about by the force between them that deforms and fractures their respective structures. This force can only oppose their relative motion, so at the point where there is no relative motion there can be no force; in other words, once the building and the aircraft are moving at the same speed, they will continue moving at the same speed. Because the mass of the building is so much greater than the mass of the airliner, that final speed is very close to the initial speed of the building.

It's all most easily expressed in terms of conservation of momentum, and conservation of momentum works in any inertial frame of reference. If we view the building as being stationary, its initial velocity is zero, and its final velocity is the velocity of the airliner, multiplied by the mass of the airliner, divided by the sum of the two masses; since the mass of the building is about 1800 times that of the airliner, the final speed of the building + airliner is very much smaller than the initial speed of the airliner - about a quarter of a mile per hour. Still, quite high for something that's fixed to the ground. If, on the other hand, we view a moving building as hitting a stationary airliner, then the final velocity of building + airliner is the velocity of the building, multiplied by the mass of the building, divided by the sum of the two masses; since the airliner is so much smaller than the building, that's very nearly the same as the initial velocity. In fact, it's about a quarter of a mile an hour slower.

So, if the plane is moving and the building isn't, the plane only moves the building relatively slowly; and if the building is moving and the plane isn't, the plane only slows down the building a tiny amount. All this simply follows from the ratio of the masses.

Dave

This stuff is fascinating. I've been watching one or two of Walter Lewin's lectures I'll have to take time with this though. hanks for the info. How would you write this in scientific notation incidentally ?

''and its final velocity is the velocity of the airliner, multiplied by the mass of the airliner, divided by the sum of the two masses''

 

[T.A.M.]

Here bill, sing along with this guy,

http://www.youtube.com/watch?v=HkJ-913CJTg&feature=related

TAM

 

[alexi_drago]

If you take the earths rotation into account. A plane flying west with respect to an observer on earth would appear to be moving backwards from the point of view of a stationary observer in space, if it were to collide with a building then it would appear that the backward flying plane was caught up and hit by the building.

But since we can be pretty sure the outcome of a plane hitting a building is going to be the same whether it's flying east or west gives us a clue that it's all relative.

 

[beachnut]

This stuff is fascinating. I've been watching one or two of Walter Lewin's lectures I'll have to take time with this though. hanks for the info. How would you write this in scientific notation incidentally ?

''and its final velocity is the velocity of the airliner, multiplied by the mass of the airliner, divided by the sum of the two masses''

The WTC tower took off at .35 feet per second after the impact of Flight 11. Where is it now? Did the WTC tower move 1000 feet before it collapsed?

 

[bill smith]

If you take the earths rotation into account. A plane flying west with respect to an observer on earth would appear to be moving backwards from the point of view of a stationary observer in space, if it were to collide with a building then it would appear that the backward flying plane was caught up and hit by the building.

But since we can be pretty sure the outcome of a plane hitting a building is going to be the same whether it's flying east or west gives us a clue that it's all relative.

There was a young lady called Bright
Who could travel much faster than light
She went out one day
In a relative way
And came back on the previous night.

 

[Hokulele]

Did you cut your signature short ?

I think you are confusing TjW with tfk.