The roof line of the North Tower of the World Trade Center is shown to
have been in constant downward acceleration until it disappeared. A
downward acceleration of the falling upper block implies a downward net
force, which requires that the upward resistive force was less than the
weight of the block. Therefore the downward force exerted by the falling
block must also have been less than its weight. Since the lower section of
the building was designed to support several times the weight of the upper
block, the reduced force exerted by the falling block was insufficient to
crush the lower section of the building. Therefore the falling block could
not have acted as a "pile driver." The downward acceleration of the upper
block can be understood as a consequence of, not the cause of, the
disintegration of the lower section of the building.