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I am having trouble understanding a building collapse. According to NIST, on 9/11 WTC7 collapsed after fire weakened its structural steel. They have however admitted that WTC7 fell for 2.25 seconds at free fall. (http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201A.pdf) However isn't all the gravitational potential energy of the building used up in attaining free fall? Why did the structure in the way not slow the collapse in any way during this period? Fire can't blow out 8 stories of structure simultaneously and continuously floor by floor. Energy and momentum don't seem to be conserved here which is a violation of the laws of physics. Can anyone explain how I am wrong here?

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