> If the floors collapse one at a time and the collisions are elastic,
> the bottom floor hits the ground a a time equivalent to the top floor
> free-falling, but the time for the top floor to hit is relatively
> delayed.  If the collisions are inelastic, then momentum is conserved
> as you say, so there is still significant acceleration, approaching
> free fall speeds as the falling mass becomes much greater than the
> mass it is colliding with (which would also be the case if 
> the collapse
> starts at the bottom of the building instead of at the top.  Perhaps
> that explains tower 7's "anomalous" fall speed.)

What if the collaps was not purely top-down but (at least partially)
bottom-up, so the first floor collapses first, the whole building falls
one floor down, now the second floor collapses, and the remaining falls
one floor down, etc. I am not sure, but I think the falling of the top
floor in this model would be almost free-fall.

Wouter van Ooijen

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