Thanks to 'Charlie Gate', many truthers, particularly in the UK, have been brushing up on their 9/11 knowledge, and there has been fierce debate on YouTube about things like the freefall of WTC7.
The apologists for NIST have been claiming that WTC7's 2.25 seconds of free fall can be explained by the lack of core support and exterior column buckling between floors seven and fourteen (depicted above) that NIST's model alleges occured. Basically they're saying it wasn't perfect freefall, but something indistinguishable - like 98% of freefall or something - which can be explained by the fact that the resistance offered by the eight crippled floors was so low that it could only slow the acceleration by a tiny amount. Let's do some simple physics to assess this claim.
As anyone with even a rudimentary understanding of physics knows, the downward force (or weight) of an object being acted on by gravity is equal to the mass of the object multiplied by gravity, or:
F = M g
When there is a resisting force acting in the opposite direction (f), the force available to accelerate the object downward is:
F = Mg - f
Since force = mass × acceleration, the acceleration of an object being resisted can be calculated by dividing both sides of the above equation by M...
a = g - f/M
Since force = mass × gravity, you can substitute mg in place of the little f, where m is the maximum mass that whatever is doing the resisting can support, giving you...
a = g - mg/M
or
a = g (1 - m/M)
In the case of NIST's WTC7 theory, the acceleration of the collapse would therefore be determined by the ratio of the mass of the upper section (M) and the mass that those eight buckled floors can support (m). Because the building is collapsing, M is obviously going to be greater than m, but how much greater?
If m is just one tenth of M - say if the mass of the upper section was 50,000 tonnes and the mass that the eight buckled floors could support was 5,000 tonnes - then the acceleration would be 8.8ms-2, which is noticeably less than freefall. If m is just one twentieth of M - say if the mass of the upper section was 50,000 tonnes and the mass that the eight buckled floors could support was only 2,500 tonnes - then the acceleration would be 9.3ms-2, which is still noticeably less than freefall. So unless the eight buckled floors couldn't even support say a fiftieth of the mass of the upper section, then the rate of WTC7's collapse, for those eight stories, would be noticeably less than freefall.
Remember, skyscrapers are overdesigned, so it's hard to imagine, even with the loss of the core structure and a buckled exterior structure, that those eight floors would lose so much strength that they couldn't even support one fiftieth of the mass above.
Another thing to consider is how sudden the transition to freefall was. Before the period of freefall, the outer structure barely moved at all, then all of a sudden it just dropped. If the free fall drop was the result of a loss of support in floors seven to fourteen, then that loss of support in those eight floors would have had to have been instantaneous. Otherwise the upper section would start crushing those floors at a rate much lower than freefall the moment they could only just no longer support the mass above - much like how the partial collapses of the TU Delft building and the Windsor tower initiated slowly...
Notice how both collapses begin quite tentatively. This is because the scales have only just tipped. The resistance is less than the weight, but only just. This is what we would expect to see in WTC7. The alleged domino collapse of the core structure and buckling of the outer structure would not have been instantaneous. So even if this could explain the freefall, it can't explain how the onset of freefall was so sudden.
Essentially, the apologists for NIST expect us to believe that eight floors lost at least a fiftieth of their resistance to the weight above in an instant.
But of course, all this is giving NIST's computer model too much credit, since the model fails to match observed reality in many ways. For one thing, the massive bulging of the east side, as seen in the south view of their global collapse simulation, was not observed.
My point is, even if NIST's computer model was an accurate depiction of the state of the lower floors, it's still unlikely that the resistance offered by those floors would be negligible enough for virtual freefall to be plausible.