On Dec 9, 2006, at 6:34 PM, tachyon_1@email.com wrote:

>  Sure, the Kanakas theory. However, most of each floor is pulverized in
> the process. Therefore a large portion of that kinetic energy that 
> would
> be transferred to the structure and floors below is used up. Whatever
> energy used in this destructive process is _not_ used to accelerate the
> next floor. And no matter how much is transferred, the initially 
> velocity
> of each floor is 0.

Momentum is still conserved in an inelastic collision (and a fully 
inelastic collision is the maximum "energy absorbed by destructive
processes, I think.)  That means that if one floor collapses onto
another (of equal mass) the velocity of the combined mass immediately
after the collisions is going to be V/2, where V was the velocity of
the first floor just before the collision.  Once the falling mass is
9x the value of the poor floor underneath, the next initial velocity
is going to be 90% of the previous velocity.

It looks to me like for a tall building, the overall fall rate is
going to approach free fall rates pretty quickly, especially if the
collapse starts other than on the top floor.  Note that the exact
mechanisms and quantities of energy absorption are irrelevant; your
overall fall speed is going to be somewhere in between that predicted
by elastic collisions and inelastic collisions; we're certainly NOT
talking about a case where one floor succeeds in bearing the load of
the fall for "a while" and then starts collapsing as a separate event.

And that's without any of the other plausible reasons for the collapse
to proceed faster than expected.

The use of seismic data to analyze the falls is ... interesting.
Does it have any history?  Have people studied seismic data for
demolitions, for instance? I can think of a lot of reasons why
results would be different than you might expect...

BillW

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