i see you added a bit since i posted last.
i think you'l find the kinetic energy of 20floors of skyscraper going even 7 miles an hour to be rather large
lets see weight of world trade center was about 450000tonne lets say top 20 floors weighed 70000tonne
now that going at 7mph
EK = (1/2)mv2
(1/2)x70000000kgx
3.12928m/s/2 = 342733766.144 joules
now you may think thats meaningless numbers lets look at http://www.mhi-inc.com/Converter/watt_calculator.htm
if you enter the 342733766.144 joules in with say a generous impact time of .1 seconds the the results you get are
3427000[FONT=Verdana, Arial, Helvetica, sans-serif]
Kilowatts
4596000[/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]
Horsepower
11700000000[/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]
BTU/hr[/FONT]
975200 tons (884 686.558 tonne)
notice the weight of 884 686
tonne thats over 12x the standing weight of the top section of wtc
thats what 7 mph of falling skyscraper can do...
and they took longer than 10 seconds to fall
in this video theres clearly plenty of floors still there at 11 seconds into the film
[video=youtube;smreRx51cus]http://www.youtube.com/watch?v=smreRx51cus&feature=related[/video]