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Originally posted by exponent
We're ignoring vertical velocity, taking purely the horizontal component, gravity will not accelerate it, so the only acceleration after ejection is friction from the air.
Originally posted by tezzajw
I agree with exponent's results.
Originally posted by exponent
If you assume that the object accelerates from rest over a period of 9 seconds, its average acceleration is as you say. But the object did not do this.
It accelerated from rest in an exceedingly short period of time, and then travelled 150m in 9 seconds
No, this is absolutely incorrect. You are not working out the 'net force' here, you are making claims about the Kinetic Energy content of the object when it has travelled 150m.
Originally posted by bsbray11
Technically, in mechanics, a force is defined as a change in momentum or also a mass multiplied by an acceleration, or F=ma.
A piece of debris weighing a ton (a modest weight compared to a lot of debris ejected from the towers), or 907 kg, traveling over 500 feet or about 150 meters laterally, takes a large force to produce.
By using a formula based on a continuous acceleration, you ignore the fact that the final velocity is marginally smaller than the ejection velocity.
They are wrong, the distance travelled is 150m, the time taken is 9s, so therefore the speed is 16.7m/s.
To accelerate the object to 16.7m/s, it takes energy 1/2mv^2 or 453.5*16.7^2 or 126KJ.
The assumption that the object continues to accelerate until impact gives it a higher kinetic energy than the true conditions, which were an initial large acceleration followed by 150m travel at a constant speed (ignoring wind resistance).
Originally posted by bsbray11
Just so I can see that you know what you're doing, I give these three variables and solve the equation for "a":
d=150m (total lateral distance traveled)
t=9s (total time elapsed)
v0=0 (started from rest)
Just so we can be clear we are in agreement about the math, that the math is correct.
Originally posted by tezzajw
The main initial, impulse force is the one that gives the matter its kick start velocity in the horizontal component. Once the mass is on it's way, it's not subjected to any further acceleration.
I agree with exponent's results.
Originally posted by bsbray11
The horizontal component in reality did not accelerate over time, but was imparted with an initial velocity from a force and then just sailed out into the air, only having air to resist it, until it hit the ground.
Originally posted by bsbray11
No, I was finding the net force, F, exerted on the object horizontally. I said this in my first post:
It is CORRECT to use that acceleration for the entire period of the 9 seconds, because NO OTHER NUMBER will accurately describe the sum of all accelerations over the entire 9 seconds. Again, what do you not understand about that? Are you reading my words? And then thinking about them?
The problem is that you used the final velocity that my average acceleration implied and used it as if it were a demonstrated fact in your KE figures. I didn't HAVE to measure the final lateral velocity, because I supplied other variables that I DID justify
I still haven't seen how you justify your final lateral velocity, because I certainly never claimed it was valid, so don't point at me. I never even looked at velocities.
Please try to understand the things I am saying before you knee-jerk another response. Just sit and think for a second.
Originally posted by exponent
You have derived the correct result for acceleration based upon your premise, but your premise is faulty.
Originally posted by bsbray11
Exponents "results" consisted of taking the average acceleration which we both admit can't be used to find instantaneous velocities or accelerations, and then trying to use it to find an instantaneous velocity.
Originally posted by bsbray11
How is the premise faulty in the actual equation?
Is it any one of the variables I use? Surely the equation itself is reliable.
Once again, I am trying to find FORCE, not velocities or instantaneous accelerations.
It's just like life, we all vote on partisan lines - don't we? - should we?
Originally posted by exponent
Thank you, although I find it funny that bsbray has so many flags for an incorrect post. Either people don't like me, people do like him, or people vote on partisan lines. That or I have committed a False Trichotomy fallacy!
Originally posted by exponent
The premise is faulty in that you claim to be able to calculate work, and work = kinetic energy, kinetic energy is dependent on velocity.
Your calculation of force is irrelevant, it has no bearing nor any real meaning that I can see. Surely you don't dispute the upper section of the towers can exert a few thousand Newtons of force?
Originally posted by bsbray11
Work is also defined by a force times a distance, and using that other equation I didn't need to find instantaneous velocities or accelerations. A constant force accelerating a one-ton mass at 3.7m/s^2 will travel 150m in 9 seconds just the same, and that whole force would be applied the whole distance as far as I can tell.
It is an abstract figure but I was mainly just trying to make a point that there was real energy behind these things flying everywhere and it wasn't just from leaning and rotating about a fulcrum.
And I have a hard time imagining what might happen inside one of the buildings that might send out a multi-ton chunk of steel several hundred feet laterally, let alone upwards of 80% of the total mass.
Originally posted by exponent
The problem is not that the distance or time changes, it is that in your equation the section is travelling at > 30m/s when it impacts, when in reality it would have been travelling at around 17m/s, as kinetic energy scales to the square of velocity, this results in your estimate being much higher than in reality.
That's a fair comment, there were quite a few large sections which hinged off, you can see some in the Bob and Bri video, but a good accounting of the collapse behaviour is extremely hard to do.
Well lets remember that the speed required is only 16.7m/s, which is under 2 seconds of gravitational acceleration.
The physical mechanism may be unintuitive, but that is simply because we never experience anything like this in our everyday life, and so we have no real point of comparison.
I calculate a required acceleration of 83.3m/s/s, and using your above equation we can find that the distance travelled is 1.67m and the force applied is 75.5KN, giving us W=75583.33*1.67 = 126KJ. Within rounding errors as before.
Originally posted by bsbray11
Ok, forget energy, and forget work, I should have never mentioned that either. The acceleration I assumed to find that force doesn't contain any specific information about real instantaneous values, only an equivalent, like I've been saying. And I intentionally used other variables that I knew to avoid having assume any instantaneous velocities. I was originally just making the point that a non-gravitational force would have to be present, some significant amount of force, and furthermore it would have to be applied to the majority of the mass of either tower.
Something else that's counter-intuitive to me is the fact that all this ejected debris didn't just come from a side where the collapses tended to lean, and where things tended to be under the more stress. It went out in all directions.
Originally posted by exponent
I still don't understand this last statement, why does the force have to be non gravitational?
The energy does get used up and the mass does get depleted as you have said, but the question is does enough energy get used to slow or stop the collapse?
The most rigorous calculations done so far are those in BLBG and this has been pretty rigorously reviewed at this point.
Originally posted by bsbray11
Even if the energy is ultimately provided by gravity, you have to admit something else is happening along the way. Whether you think it's a rotation that abruptly ends but some chunks break loose and keep flying, or the debris slides down an incline that gives it lateral momentum, it isn't purely gravitational energy at work, there are other things involved. And as those "other things" are needed to explain more and more of the total mass being thrown out, I think the possibilities become more limited, but nonetheless I think it defeats investigation to just call it all gravity from the start.
Shouldn't any loss of energy slow it by definition?
For some reason I doubt they took mass being lost over the sides into account, or the energy required to displace all that mass laterally. But I could be wrong.
I know Greening never took that into account in his earlier 1-dimensional arithmetic exercise of a model. Either way the whole process boils down to what you put in, what you leave out, and how much energy you think what you put in should really take up, and they play with a lot of room in those numbers.
posted by jprophet420
The NIST report did not in fact determine the cause of the collapses. It determined that the pancake collapse could have been initiated under the circumstances with manipulated data.
posted by mmiichael
I'm a spokesman for NIST, but don't find their revisions suspicious nor do I think there was withholding of evidence to conceal significant data.
Originally posted by SPreston
posted by mmiichael
I'm a spokesman for NIST, but don't find their revisions suspicious nor do I think there was withholding of evidence to conceal significant data.
Well that certainly explains a lot and is not a bit surprising. Many government loyalists seem to be spokesmen for the 9-11 perps also.