North Tower Acceleration

[cpwm17]

46. I didn't say they fell at free-fall speed

There was some resistance to the collapse from the strength of the structures themselves. The buildings absorbed some of the energy of the collapses. There was less resistance between the floors.

As the towers fell they collected more mass from the lower sections. This mass contributed to the collapse further down the building. But this mass collected from the lower sections started from a zero velocity, so this mass slowed down the collapse due to conservation of momentum.

For the upper portion to EXERT THAT FORCE would REQUIRE IT TO DECELERATE. IF IT'S NOT DECELERATING, IT'S NOT EXERTING A FORCE. Because there's nothing to exert it against.

The towers would only be guaranteed to decelerate if gravity disappeared during the floor collisions. Gravity is always there, and gravity is what is accelerating the towers downward in the first place. You have to analyze the relative forces to determine if the collapse decelerates or continues accelerating downward.

Dropping an object from a dozen feet (approximate distance between floors) exerts many time the force than if you just placed the object gently. The towers weren't design to support a force many times the static weight of a multi-floor section of the buildings. A complete collapse was guaranteed once it began.