The physics toolkit

[tfk]

,,,,,,,, I have made a similar point several times now in this thread.

All femr has been able to do is put a finer detail to the behaviour of the structure just prior to and through the fall of the north face.

He has also managed to illustrate that none of this finer detail can in any way aid in deducing exactly what caused the progression of the collapse to become a global collapse.

Indeed it also illustrates one reason WHY NIST did not bother to conduct a more thorough analysis of the final global collapse, it would not have, nor could it have, made any difference to their conclusions.

BTW, in the whole suddenly applied force on a thread bit. The glaring simplification of no friction force was the first thing that 'sprang' to mind.
The greatest friction force due to air resistance would occur as the downward motion reachs a maximum. This would occur very soon(dare I say , as soon as) the extra mass is applied. It would quickly lessen as velocity dropped, reach zero as velocity reached zero and then become a downward force as the thread rebounded and everything moved upward. The definition of a damping effect, and one reason why in first year physics labs an experiment that has the student slowly apply more mass to a thread in order to deduce the Young's Modulus of the thread material. No significant, overshoot, or oscillation, or damping forces to make such a calculation difficult. I do recall that we had to time how long it took for each stretch to finish( the time to reach a new equilibrium)

There is however only a 'thin' application of this concept to the subject of the thread(that is the forum string of posts which we refer to as a thread)

Secondly, some have wondered if this new force was constant. Well if this force is due to a suddenly applied, added mass that is not removed at some point then yes, of course it is constant since the extra force is due to the added mass.

This whole problem was so weirdly phrased that I've skipped it.

But JD, I don't think you meant that the air friction was a max when the downward motion reaches a max. At that point, the velocity will be zero (unless the string has broken) and the air friction will also be zero.

Also, I don't believe that the upward force on the weight (which equals the downward force on the string) is a constant. It is set by the stress-strain curve of the string, and is independent of the weight. (Except that the weight sets the max value.)

If the elongation is in the elastic range, it'll be an approximately linear force vs. string deflection. If the string is in the plastic range, it'll be whatever force / deflection curve applies there.

tom

 

[Miragememories]

You're using an equation of motion that is based on assumptions that do not apply in the real world. The actual equation of motion is:

mu'' + cu' + ku = p(t) where u is displacement

The post you've linked to has set c = 0. Femr2 has given c (the viscous damping coefficient) a definition and a reasonable one at that. I don't know if he arrived at that through investigoogling or if he took a graduate level engineering course in Dynamics of Structures. I would guess it is the former as he is unable to provide a rational defense of his answer except linking to papers.

Unfortunately for you, I've taken those classes, and used the knowledge garnered in them in my profession. Which means when you say that he's 100% wrong, I know that you're full of . He tried to apply real-world mechanics to a complicated problem. You claim he doesn't understand physics because he made a different assumption and a perfectly valid assumption no less. This would be akin to you inquiring what the distance of a block was after travelling for 10/s and having an initial velocity 10 m/s and then claiming he was 100% wrong because he made an assumption on what frictional forces that block was under. Based on that, it's safe for me to assume that you don't know what the you're talking about or that you're intellectually dishonest.

And you still believe that the frictional heat of the collapses compared to that of a shuttle re-entry Newtons Bit??

What physics calculations did you employ for that comparison?

Retirement must be nice.

MM

 

[Newtons Bit]

This whole problem was so weirdly phrased that I've skipped it.

But JD, I don't think you meant that the air friction was a max when the downward motion reaches a max. At that point, the velocity will be zero (unless the string has broken) and the air friction will also be zero.

Also, I don't believe that the upward force on the weight (which equals the downward force on the string) is a constant. It is set by the stress-strain curve of the string, and is independent of the weight. (Except that the weight sets the max value.)

If the elongation is in the elastic range, it'll be an approximately linear force vs. string deflection. If the string is in the plastic range, it'll be whatever force / deflection curve applies there.

tom

The problem was for an elastic thread. It doesn't exactly exist in the real-world, but trying to set up equations of motion with a non-linear stress-strain curve would be... fun.

 

[Miragememories]

The problem was for an elastic thread. It doesn't exactly exist in the real-world, but trying to set up equations of motion with a non-linear stress-strain curve would be... fun.

Anything other than reality apparently.

MM

 

[jaydeehess]

This whole problem was so weirdly phrased that I've skipped it.

But JD, I don't think you meant that the air friction was a max when the downward motion reaches a max. At that point, the velocity will be zero (unless the string has broken) and the air friction will also be zero.

Tom, I do not understand this.

Air resistance/friction is proportional to the square of the velocity of the object. The downward motion is, by definition, the velocity of the object(direction and magnitude of motion). Thus when velocity is at a max the friction is also at max. There will be a slight lag between application of extra mass and maximum velocity, it does have to acellerate from zero.

The very act of the thread stretching introduces other forces that act against this stretch(rearranging of the molecular structure of the thread).

 

[jaydeehess]

And you still believe that the frictional heat of the collapses compared to that of a shuttle re-entry Newtons Bit??

What physics calculations did you employ for that comparison?

Retirement must be nice.

MM

MM, do you have anything at all to add to THIS thread?
There have been several topics raised, do you have a comment on any of them or is this a hit and run at big bad ol'Newton's Bit?

 

[jaydeehess]

Lastly, are we all agreed that no matter the amount of finer detail that anyone can pull out of the video of the final , global collapse of WTC 7, what one will not get is evidence of the use of explosives to bring about this collapse?

 

[Newtons Bit]

MM, do you have anything at all to add to THIS thread?
There have been several topics raised, do you have a comment on any of them or is this a hit and run at big bad ol'Newton's Bit?

It's generally best to ignore people as irrelevant as MirageMemories. He's incapable of having a discussion without ad hominem.

 

[Newtons Bit]

Tom, I do not understand this.

Air resistance/friction is proportional to the square of the velocity of the object. The downward motion is, by definition, the velocity of the object(direction and magnitude of motion). Thus when velocity is at a max the friction is also at max. There will be a slight lag between application of extra mass and maximum velocity, it does have to acellerate from zero.

The very act of the thread stretching introduces other forces that act against this stretch(rearranging of the molecular structure of the thread).

The damping ratio that femr2 used is based on material properties, not on friction of an object moving through air.

 

[jaydeehess]

The damping ratio that femr2 used is based on material properties, not on friction of an object moving through air.

ok, I see his point now.
The first thing that sprang to my mind was friction force of air resistance as a damping effect.

Forces due to the rearrangement of the structure of the thread would be much different.

 

[tfk]

Tom, I do not understand this.

Air resistance/friction is proportional to the square of the velocity of the object. The downward motion is, by definition, the velocity of the object (direction and magnitude of motion). Thus when velocity is at a max the friction is also at max. There will be a slight lag between application of extra mass and maximum velocity, it does have to acellerate from zero.

The very act of the thread stretching introduces other forces that act against this stretch(rearranging of the molecular structure of the thread).

JD,

Ahhh that explains it. I knew there had to be some simple misunderstanding...

I took "downward motion" to mean "downward displacement", which at max would have the velocity = 0.

You meant "downward motion" to mean "downward velocity".

Simple definition of terms...

Tom

 

[femr2]

Femr2 has given c (the viscous damping coefficient) a definition and a reasonable one at that.

There's loads of other factors of course, but the damping ratio is (as far as I'm aware) the easiest way to express quite a few of them.

I ran into the problems of system definition (and dusty brain ) a while back with the (seemingly infamous) CoM energy loss gaff, so when bigc/carlly pasted the question a year or so ago, I gave a ranged answer.

It would be useful for someone other than myself to assert that Fo/2 is an unattainable limiting case to close the book on the whole ramble.

 

[Newtons Bit]

There's loads of other factors of course, but the damping ratio is (as far as I'm aware) the easiest way to express quite a few of them.

I ran into the problems of system definition (and dusty brain ) a while back with the (seemingly infamous) CoM energy loss gaff, so when bigc/carlly pasted the question a year or so ago, I gave a ranged answer.

It would be useful for someone other than myself to assert that Fo/2 is an unattainable limiting case to close the book on the whole ramble.

I agree that the Fo/2 is a limiting case that only occurs when c = 0. You won't find that with any real-world material.