On Dec 6, 2006, at 3:39 AM, Gerhard Fiedler wrote:

> I don't know how much that would slow the whole fall, following
> this scenario. Calculating that isn't complicated (using these
> implified conditions); my feeling is that the slow-down wouldn't
> be dramatic, because it's only a real slow-down at the first few
> hits where speeds are still slow. Later on, with higher speeds,
> the falling mass is already much more (several floors) than the
> one floor mass they bump into, so the slow-down on each successive
> impact will be less (as a fraction) than on the impact before.
>
I nearly think that you are agreeing with me!

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.)

BillW
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