World Trade Center Demolition Acceleration
Downward Acceleration of the North Tower: http://www.youtube.com/watch?v=xG2y50Wyys4
Race With Gravity: http://www.youtube.com/watch?v=eXSHm3CdHf4
David Chandler North Tower Exploding: http://www.youtube.com/watch?v=dtx_GcFCs6c
Video analysis, using Physics Tool Kit software, of the speed of the demolition wave moving down a face of the North Tower in comparison with the rate of freefall through air of heavy material nearby. (Analysis based on the BBC footage which can be seen in full here ) source
Towers "came down essentially in free fall, as seen in videos” ."  S. Sunder, W. Grosshandler, H. S. Lew, et al. “National Institute of Standards and Technology (NIST) federal building and fire safety investigation of the World Trade Center disaster, answers to fre- quently asked questions”, Gaithersburg, MD: National Institute of Standards and Technology, August 30, 2006. [Online]. Available: NIST, http://wtc.nist.gov. [Accessed March 17, 2008].