The hijacked jetliners might have come close to toppling the World Trade Center Towers on impact on Sept. 11, according to new calculations by a Swarthmore College physics professor. This counters the views of most experts and a recent federally funded engineering study that concluded the planes lacked sufficient force to knock the towers down on impact.
"Certainly the loss of approximately 2,830 people in a single event is a tragedy," says Professor of Physics Frank Moscatelli, a native New Yorker. "But assuming an occupancy of 40,000 to 50,000 people in the towers alone at the time of impact, we could have had a catastrophe well beyond what we actually experienced last September."
The federal report, released last month, concluded the buildings could have remained standing if not for the enormous fires that broke out after they were struck by the hijacked airplanes. The report, sponsored by the Federal Emergency Management Agency and the American Society of Civil Engineers, supports statements made on the recent NOVA special Why the Towers Fell.
But Moscatelli contends that the report's conclusions and similar expert opinions err by focusing solely on force. "You must also compare the torque, which is a physical measure of 'twist' produced by the planes with that due to the wind load the towers were designed to withstand," Moscatelli says. "Comparing the static weights of the buildings and planes is wrong. The planes were moving, and that clearly changes the problem. The buildings did not have to bear the weight of the planes; they had to stop the planes."
Moscatelli calculated that the torque applied by the planes' impact -- 7.7 million ft. tons -- actually exceeded the amount the towers were designed to resist due to wind load -- 7.4 million ft. tons. "So they could have immediately collapsed, if not for the fact that neither object is a rigid body and that the towers flexed quite a bit upon impact with the planes," he says. "If they had not at least bent temporarily, they would have been in danger of instantly toppling."
Moscatelli also determined that the 11,000 tons of force required by the towers to resist the wind barely exceeded the 7,000 tons of force required to stop the planes. "In fact, the stopping force for the plane scales as the square of its velocity, so if the plane was traveling at 564 mph these forces would be equal," he says. "This is probably why the terrorist pilots flew at such an uncommonly high speed for that aircraft, at that altitude, for that particular maneuver. They flew as if they wanted to knock them down, and I think we cannot conclude that they were so far off from doing just that."
Moscatelli previously calculated that of the three sources of energy delivered to the twin towers on September 11 -- exploded jet fuel, kinetic energy due to the motion of two aircraft, and gravitational potential energy due to the falling building material -- the last was the most devastating. This is supported by the extensive damage caused to surrounding buildings, as noted in the federal report.
"My calculations show that the largest component by far was the latter," Moscatelli says. "This is due to the large mass and height of the towers. The airplanes destroyed 20 stories of the buildings, and gravity did the rest. Their splendor was their undoing."
Located near Philadelphia, Swarthmore is a highly selective liberal arts college with an enrollment of 1,450. Swarthmore is consistently ranked among the top liberal arts colleges in the country.
Materials provided by Swarthmore College. Note: Content may be edited for style and length.
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