Comment: Progressive collapse is the

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Progressive collapse is the

Progressive collapse is the phenomenon which generally causes a small failure to bring the entire structure down with it.

When a failure in one structure affects another structure, some momentum energy is transferred from the first to the second. This slows down the collapse to much less than free fall.

In free fall acceleration, there is nothing below to slow down or absorb any energy. This is what we saw in WTC7. Free fall was achieved with explosives which continuously removed what was below the falling structure, so as not to slow the collapse down.

If a failing structure brings down the entire structure, as you suggested, you will not see anything near free fall.