The Big Dog
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- Jul 26, 2007
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if the camera is near to, or looking obliquely up at, the building (as camera 3 is), then you have to be careful to compensate for non-constant scaling factors and horizontal vs. Vertical components of motion.
For camera 2 (the cbs "dan rather" video), the camera is far enough away from wtc7 & is looking pretty much square onto the building that these two effects are inconsequential.
When it comes to horizontal vs. Vertical motion, then you will only get the vertical component of motion from the one camera. A multiple camera analysis could, in principle & practice, resolve the 3 dimensions of motion.
But the horizontal & vertical equations of motion for a free falling object are independent of each other. The old "drop a marble at the same time you shoot a rifle bullet & they hit the ground at the same time" effect.
Of course, the north wall is not a "free falling object", but is tied to the collapsing structure behind it at 1000 different locations.
It looks pretty evident from the videos that the upper east quadrant (about 1/3rd horizontally and vertically) of the north wall peeled away from the building, and fell to the north.
But the point that nist followed was not in that section, but more towards the center / top of the parapet wall.
It looks clear to me, based on femr's data, that the point that he selected (the north west corner in the data plotted below) descended at a non-constant acceleration throughout its visible fall, as shown by the green line here.
[qimg]http://img839.imageshack.us/img839/9636/ffavswtc7northwallaccel.png[/qimg]
there is still an unanswered question of a scaling factor. This would shift the magnitude of the curve up or down. But it would do nothing to change the shape of the curve.
As i've shown, a constant free-fall acceleration, starting at the moment the north wall began to move (in this chart at t = 3.5 seconds, unrelated to any of nist's timing), would match the red line.
There is no period of time that the green curve had a constant -32.2 ft/sec^2 acceleration.
From the chart, you can see that between 5.0 & 6.6 seconds (about 1.6 seconds), the average was about -32.2 ft/sec^2.
If the scaling factor is slightly higher, then this would extend the time over which the average is -32.2 to a number closer to nist's 2.2 seconds.
A small point that is interesting to me, but utterly irrelevant to the question of a controlled demolition, is that it seems unquestionable to me that, for a brief period of time, the roof line point followed does actually exceed a "g" acceleration.
This seems incontrovertible from both nist & chandler's data as well. If it really fell "at g", then all the velocity points would be (nearly) exactly on the straight line velocity graph. (within measurement error, of course.)
looking at all the data points, they aren't really that close to the curve.
Contrary to some folks' comments on this issue, this "greater than g acceleration" does not violate any of newton's laws. Precisely because the north wall is not an isolated object being acted upon only by gravity. There are other forces acting on it. Forces that can, and certainly appear to, act in a downward direction. (the specific forces being debris falling on lower structures that are attached to the external wall.)
tom
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