The visible descent NIST analyzed
NIST's WTC 7 FAQ addresses a video that appears to show the building descending in free fall. NIST says its draft report described about 18 stories of visible north-face descent in 5.4 seconds, 40 percent longer than a 3.9-second gravity-only descent. [1]
NIST tracked a point near the center of the north-face roofline from first perceived vertical motion until the face left the video. The 5.4 seconds describe that visible descent, not the whole building collapse. [1]
The three acceleration stages
NIST reports three stages within the 5.4 seconds. From 0 to 1.75, acceleration was below gravity; from 1.75 to 4.0, it matched gravitational acceleration, or free fall; from 4.0 to 5.4, acceleration fell below gravity again. [1]
NIST associates Stage 1 chiefly with buckling exterior columns in the lower north face and says this explains most of the extra descent time. Stage 2 showed negligible support from below, consistent with its model of columns losing load capacity. Stage 3's reduced acceleration is attributed to resistance from collapsed structure and debris. [1]
What the measurement establishes
The measurement establishes the motion of one tracked north-face point during a defined interval. It underpins NIST's Stage 2 free-fall account, but the three stages do not describe the whole collapse; buckling and resistance belong to its structural interpretation. [3]
NIST fitted a smooth displacement function, differentiated it for velocity, and also used a central-difference approximation. Figure 3-15 shows the fitted curve, computed points and a linear regression over roughly 1.75–4.0 seconds. [3]
OpenStax defines free fall as ideal motion with negligible air resistance and friction, in which objects fall with the same constant gravitational acceleration, about 9.81 m/s² (32.2 ft/s²). This is general context, not corroboration of NIST's WTC 7 measurement. [2]
FAQ 32 does not state frame rate, pixel scale, tracking error or formal uncertainty bounds. [1]