by Oystein » Sat May 19, 2012 12:43 am
Just to give you an idea about the magnitudes of energy we are talking about here:
- Each tower started with a potential energy relative to ground level equivalent to something like 120 tons of TNT
- After the collapse was finished, all of that energy had been transformed, in part via kinetic energy, into destruction, some heat, and a small portion was lost to planet earth through seismic waves (which in turn became heat eventually)
- It isn't agreed upon with precision how long the collapses took. Longer than freefall time, but probably less than double that time. So, on average, the towers collapsed at 2/3 of g. This means that, during the collapse, 2/3 of the potential energy was turned into kinetic energy (which dissipated upon hiting the ground), and the rest, 1/3, was already dissipated as destruction and heat during the collapse. This 1/3 is equivalent to some 40 tons of TNT.
Be careful, these numbers are rough and dirty. don't kill me if the "true" equivalent is 20 tons or 60 tons. This is, again, just to give you a feel for the dimensions.
Now, in a carefully crafted actual highrise CD, much less explosives are used. The Landmark Tower in Fort Worth, 115 meters high and perhaps 1/16 the volume/mass of a twin tower, required only 365 pounds of high explosives. The twin towers then might have required 2 to 3 tons of high explosives for total collapse (by far most of the energy in CDs comes from the building's potential energy, not from explosives, by the way). 40 tons equivalent were actually dissipated - obviously more than enough.
