Friday, September 10, 2010

Back to high-school: today, physics 101

My friend L. has just emailed me this video, which I new about, but which I do want to post here for those of you who say that they are not engineers and that they have a hard time coming to a conclusion about what happened with WTC7.  In this video David Chandler, a high-school physics teacher, uses basic (high-school level) yet absolutely incontrovertible laws of physics to prove tat WTC7 did fall at free-fall acceleration for a significant part of its collapse.

For those of you who, like myself, were getting bad grades in physics in high-school, let me begin by some very simple reminders:

a) "free fall" is not a speed, but the *acceleration* of a body pulled down by earth's gravity when it encounters no resistance at all.

b) if a body encounters even minimal resistance it will slow down and not fall at free fall accelerations.  For example, if you drop a bowling call though a few glass windows stacked up horizontally on top of each other, it will look like the bowling ball just smashed through without slowing down but this is an illusion.  If you carefully measure the fall of the bowling ball at each point of its descent you will detect a slight change in motion each time the ball hits a glass window.  What happens is that some of the (so-called "potential") energy of the ball is used to actually smash through the glass, thereby slightly slowing down the ball.  Conversely, and this is important, if you detect no such slowing down, that means that no resistance was encountered by the ball on its descent and that means that somebody or something already smashed the glass windows before the ball passed through that section of space.

Now take a look at Chandler's video:

What Chandler is proving here is that for a substantial part of its collapse the top of WTC7 was falling at an acceleration which is so close to the earth's natural acceleration (9.81 m/s2) that we can say with total confidence that it encountered no resistance or, if you prefer, that the weight (potential energy) of the top portion of the building was *not* used to smash through the lower floors simply because by the time the collapse began these floors had already been removed. 

How can somebody remove entire floors of a large building a) simultaneously b) symmetrically and c) instantaneously ?

There is only one way to do that: explosives.


The Saker