David chandler:
“Free fall can only be achieved if there is zero resistance to the motion.”57 In other words, the upper portion of Building 7 could have come down in free fall only if something had suddenly removed all the steel and concrete in the lower part of the building, which would have otherwise provided resistance (to make a considerable understatement). If everything had not been removed and the upper floors had come down in free fall anyway, even if for only a fraction of a second, this would have been a miracle – meaning a violation of physical principles. Explaining one of the physical principles involved, Chandler said:
“Anything at an elevated height has gravitational potential energy. If it falls, and none of the energy is used for other things along the way, all of that energy is converted into kinetic energy – the energy of motion, and we call it ‘free fall.’ If any of the energy is used for other purposes, there will be less kinetic energy, so the fall will be slower. In the case of a falling building, the only way it can go into free fall is if an external force removes the supporting structure. None of the gravitational potential energy of the building is available for this purpose, or it would slow the fall of the building.”