Abstract: 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.