http://www.debunk911myths.org/
http://www.jod911.com/
PE= m *9.8*413= 4073m
Mass is, by definition, simply a measurement of how much force a given amount of material will exert AT REST on an object that is resisting gravity.
SOOOO
The top floor would hit the ground with the same force as a 4073 story building, if that fall was unimpeded.
Now let's consider the fall of the top 30 stories.
They fell through the 3-5 floors of damaged sections and impacted the building below with some amount of force.
Let's call the distance accelerated as 3 floors and be generous. This is 11 meters.
Acceration of an object for 11 meters at 70% of gravity(dan's figure), would yeild an ending velocity of:
v^2= 2ad=2*9.8*.7*11=150=v^2, find the square root of 150, and bada bing, you get 12 meters per second
Subs ute this into the kinetic energy equation:
ke= 150*.5*m=ke=75m
This means the 30 floor section impacts the undamaged portion with the kinetic energy of SEVENTY FIVE TIMES ITS MASS.
Think about this for a moment.
The lower section of the building is designed to hold that 30 stories stationary plus a safety margin of 10 or 20%. So the maximum force that the underlying structure could apply to that falling section is 1.2 times its mass.
Further:
That falling section having as much kinetic energy as 75 times its mass means that it is effectively applying the same amount of force at the impact point that a 2270 story building would. if you held it stationary. (simple math: 30*75)
For the statement "the building would not have collapsed without explosives" implies that the building could have been TWENTY TWO TIMES TALLER THAN IT ACTUALLY WAS without collapsing.
STILL FURTHER
Your calculations seem to imply that the building structure below could absorb 30% of the falling energy.
IN JUST THE FIRST 11 METERS OF A 400 METER COLLAPSE THERE IS 62 TIMES THE AMOUNT OF FORCE REQUIRED TO COLLAPSE THE BUILDING.
Your assumption of about 1/3 the energy used to collapse the building is about 20 times what is reasonable. (1/62*20= 1/3) (more actually, if you consider the further distance and mass)
What happens, then when MORE mass is added AND accelerated?
Even if half the mass falls away or off to the side, there is still FAR more force and energy than would be needed to collapse the building WITHOUT ANY EXPLOSIVES.