Mass is constant and cannot increase and in this case it actually decreased as debris was ejected laterally and dust blew away in the wind, therefore both force and acceleration should have also deceased as the kinetic energy was absorbed by the lower floors. While the force increased initially as a result of increase in acceleration, this force was countracted by the equal and opposite force of the lower intact floors according to Newton's basic laws of motion. Newton's 3rd law of motion states:<quoted text>
"Average speed" is meaningless.
After 13 seconds of "free fall" - acceleration due to gravity - a body is going 416 ft /sec or 330MPH.
Any "resistance" from the next lower floor is overcome practically instantaneously as the falling upper structure picks up speed and accummulates more mass (therefore F is growing exponentially). Buildings are designed to withstand a mostly static vertical load and reasonable "live" load, not a sledgehammer.
I think you got hit on the head by a falling bowling ball. Go back to smoking pot and reading Chaucer and leave the higher mathematics to your betters.
When a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction.
Which means the equal and opposing force of the intact lower floors should have halted the progressive acceleration of the upper mass of debris and dust falling on it.