On Dec 4, 2006, at 11:50 PM, Gerhard Fiedler wrote:

>> Others have said that the weight of the top floor would accelerate the
>> lower floors and account for the rapid fall. That is an interesting
>> point, which may very well be correct. I don't know. I would love to
>> hear from someone who does.
>
> If it falls from the top, the weight of the top ceiling crashes on the
> floor below. There it "shares" its energy and momentum, and both 
> continue
> to fall, however slower as if there were no floor below. And so on. To
> fall nearly like free fall, I'm pretty sure the whole steel frame has
>  to crash at nearly the same time (between all floors).
>
You know, I don't think I trust people with advanced physics degrees
to do what is essentially advanced mechanical engineering.  There's
just too much ... distance between theoretical mechanics and real
world situations, and for that matter, too few physicists that have
any focus on mechanics (THAT'S just for Freshman!)  Just
what IS the mechanism by which floors underneath "slow down" the fall
of the upper floors.  I seem to recall collisions of two sorts; elastic
and inelastic; I don't recall that either results in dramatic slowing
of the objects.  For that, you'd need something that absorbed massive
amounts of kinetic energy into internal deformation and heating, like
the crushable bumpers in cars.  I don't think normal building materials
do that very much, including steel frames.  Subject them to loads beyond
their yield strength, and they just break without absorbing significant
energy; why should that slow anything down?

There was an article on the physics of karate in Sci American back in
the later 70s or early 80s.  One of the interesting results/claims was
WRT that stack of concrete blocks that experts were breaking through
with bare hands; it seems that if things are set up properly, you only
have to hit the stack about hard enough to break the first slab; the
pieces of that slab do most of the work of breaking the second slab,
and so on.

(Another scenario: what's to say that a major impact like the collapse
of an entire floor isn't sufficient to significantly buckle the steel
frame of the entire building so that it no longer has any role to play
in the collapse of the building.)

I'm not claiming that I can match any of these explanations up to the
behavior people see on the tapes, but they don't seem that far off.
There's lots of stuff that operates well within the realm of physics
that is not very intuitive...
    http://www.physlink.com/estore/cart/item_images/63_xl.jpg

BillW
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