A basic graphic version of the proof done in Excel
The following graph comes from an Excel program run of a collapse of a building that is 1200 feet high with 100 floors each spaced 12 feet apart. This is similar to my own model of WTC1 except WTC1 was 110 floors and went to 1368 feet. Here are the graphs of two parameters (speed and time) of two versions, one with the collapse from the top of the building and the second of a block of 10 floors falling on the rest of the building:
The red line is the graph of the elapsed time of an object allowed to fall in free fall from 1200 feet to the ground at sea level. The blue line is a graph of the time for the roof to fall the 1200 feet while impacting floors below it and collecting them on the way down. As each floor or collection of floors impacts the next floor in the way on the way down, the increase of the falling mass slows the collapse forcing it to take more time to reach the ground. The difference in free fall and the fall with floors in the way is over 50% greater than for free fall.