I have done the maths grahod. I posted it a while ago. What more do you need to know anyway?
Free fall in a vacuum from 415m is 9.2 seconds. One of the towers fell in approx 15 seconds. Thus, the power of gravity only had some six seconds worth of energy to pulverise that building according to the law of conservation of energy. That is a lot of building in only six seconds to turn to dust.
