An analysis of the South Tower collapse is made by examining the earlier
stages of the collapse,
with careful consideration given to the conservation of energy and
momentum in the top segment of the tower. This includes
events such as the upward movement of the corner of
that top segment on collapse initiation, at the same time
that squibs appear below the segment. Information gained on the details
of that development is used to calculate the minimum energy and power of the
sources of that collapse initiation, and it is shown that the
sources of that energy and power include 2 separate sets of conventional
explosions inside the building just below the top segment.
It is shown these explosive forces
must produce the energy- and momentum-imparting moves of that top segment,
and result in the white clouds that are seen to arise from
pulverized concrete below the the floors where the plane impacted, and the
subsequent gray clouds that result from pulverized concrete (part of it black
carbon produced by the fires) of the top segment. The development of the gray
clouds is shown to result from the rapid disintegration of this top segment
near its interface with the floors below the impact.
Where did the massive amounts of energy necessary to bring the South Tower down come from? That is just one of the questions not answered in the National Institute of Standards and Technology (NIST) numerous volumes of study as to what caused each of the Twin Towers to collapse, which ascribes the collapses as caused by the plane collisions and resulting fires. However, the plane that impacted the South Tower almost missed the building, hitting near the southeast corner, with much of its fuel spilling out into the open air and creating large flames in the air as it exited near the norhteast corner. One of its engines and parts of its fuselage and landing gear crashed into other buildings away from the towers. The plane had collided into the 78th thru 85th floors of the South Tower, and the collision causing fires in the lower portion of the upper 33 floors of the tower.
The South Tower was the first of the 3 buildings of the World Trade Center that underwent complete collapse, and it did so 56 minutes after the plane collision. In Figure 1 is a view of the fires on the top floors after 56 minutes of burning, just seconds before the tower collapse started. It is a shuddering sight, knowing that people were trapped on those upper floors, but the fire does not appear to be any threat to the building as a whole.
Figure 1. The South Tower (left-hand side of the photo), 56 minutes after being struck near the SE corner by a plane, with fires on the 78th floor and above.
Compare that with Figure 2, taken just over 4 seconds after Figure 1. In that 4 seconds multiple white-colored squibs have burst out of 2 floors on one side of the tower. Squibs (as I am using the term here) are sudden high-pressure ejections of material directly out the sidewalls of the towers in a generally horizontal direction. The squibs in Figure 1 show a sudden rise in pressure along 10 or more open paths inside the building, creating the large gradients necessary to drive the squibs through the outer walls of the tower.
Figure 2. The South Tower seconds after Figure 1, in which multiple squibs have appeared from floors just below the fires. Note that the top 34-floor segment of the tower has developed a lean to the left and into the page, as seen on the right corner front face. The squibs are evidence for explosions below that top segment, which raises the question as to whether part of that that top segment has started moving up at one side.
In Figure 3, the photo 0.5 secs later shows that the fronts of these squibs have expanded out at over 30 feet (for reference, the Twin Towers are 209 feet wide). That high velocity shows significant gradients of pressure inside the building are driving these squibs.
Figure 3. South tower 0.5 secs after Figure 2, in which the squibs have expanded over 30 feet outside of their ejection region. Note that the leaning of the top tower has increased, suggesting further movement up at that corner. Note also the horizontal line running to the left of the right corner well over 100 feet above the squibs, which may be a crack in the building.
Notice the lean of the top 34-floor segment of the tower that has developed in Figures 2 and 3 just as the squibs appear below it. This lean, which can be seen video and photo sequence links at , has grown in Figure 3 to about 9° . The explosion force below that top segment can push up on the segment, and evidence can be seen for motion up in a separate video of the other (leftmost) corner in Figures 1-3. The video shot from southeast of the Tower with links at  shows a specific crack developing in the left corner 5 seconds into the video when those squibs appear, with that crack rapidly expanding a few feet upward in 1 second (see Figure 4). In 4c, in which part the smoke and dust on the right-hand side of that corner partially clear, the crack can be seen running at least 50 feet across the east side of the Tower, and also extending on the dark south side of the corner. Note there may also be one or more cracks or openings above this which would be obscured by the smoke.
Figure 4a. View of South tower collapse from South-East perspective, which shows a crack developing in the corner, while material ejections are coming out the wall(s) of the tower at levels just below.
Figure 4b. View of collapse 0.25 seconds later, which shows the crack in the corner has enlarged several feet, with the material ejections growing rapidly a couple of floors below the level of the crack.
Figure 4c. Last view of photographed of the crack 0.75 seconds later, which shows the further-enlarged crack at the left corner. There is also an extension to the right (just past the dust cover), indicating the crack runs at least 50 feet along this east side of the tower. The right of that is hidden by the smoke and dust above, and the may actually run all the way to the right corner. Notice that the crack also extends on the dark south side (left of the left corner).
The crack in Figure 4 is developing in the left corner, near where the plane first impacted just to the left of that corner on the south side. The top segment of the tower in Figures 2 and 3 is leaning away from this east side (see also Figure 5, which shows the top segment is moving away from the camera and not toward it). However, the east side is completely covered with smoke except near the left corner in Figures 4a and 4b. In 4c the smoke clears over part of the surface to its right, and shows the crack extends at least 50 feet to its right. The leaning of the tower away from that east side in Figures 2 and 3 strongly suggests that crack may extend across, possibly to the right corner.
Moving part of the top 34 floors up requires a sizeable piece of energy and momentum. Applying 2 basic principles of physics, conservation of energy and conservation of momentum, that tells us that the source of all of that energy and momentum of motion has to be the same as the source of the squibs ejecting below those top floors. These principles of energy and momentum (linear as well as angular) conservation in fact completely unravel the explanation of this collapse from plane fires as asserted in the NIST Reports on these disasters. As Figures 2 and 3 show, the collapse of the South Tower started with the appearance of multiple squibs out of 2 floors just below the upper segment fires, and almost at the same time there was the sudden move upward of that top segment at the corner as seen in Figure 4. The conservation of momentum says there must be a source for that sudden momentum upwards, and the conservation of energy says there must be a source for the kinetic energy upward, which opposes the downward gravitation force. These are large quantities of momentum and energy, and the only possible source for the energy and momentum is inside the towers at the lower floors where the white squibs are ejected from. It could not come come from the fires in the top segment which are above the surface that is being raised, or from gravitational collapse, which would in contrast cause the top segment to come down.
Figure 3 shows separate grayer clouds arising near the northwest corner near where the plane-collision exit and explosion left openings in the building. These gray clouds have appeared later than the white clouds from the squibs below, and are coming from the lower part of the top segment. Figure 5, which was shot 1 sec after Figure 3, shows the gray clouds have grown considerably in size within that second, and amazingly the top of the South Tower has come down a few stories. There has been no change in the position of the expanding white cloud, and no evidence the lower segment has changed in height. The lowering of the top of the tower is exclusively caused by the reduction in height of the top segment, indicating the top segment is disintegrating at its lower part where the gray clouds are growing.
Figure 5. South tower 1 second after Figure 3, in which gray clouds coming from the lower part of the top segment have quickly grown to a large size, at the same time at which the top segment has moved down in height. The top segment is clearly disintegrating at its bottom. Note the the width of the top of the tower appears slighly smaller than the width of the base of the tower, which indicates it is moving away from the camera. Thus the top is moving southwest in the lean.
In Figure 6, about 0.5 sec after Figure 5, that gray cloud has expanded and the top of the tower fallen by a few stories more. Furthermore, it shows another set of multiple squibs that have appeared outside the walls of floors just below the top segment, near the floors where the previous squibs came. When the second burst of squibs appears, a new phenomenon occurs, in which the top 34-floor segment of the South Tower suddenly makes a strong twist counter-clockwise. This twisting would clearly violate the conservation of both linear and angular momentum unless a large external force caused it -- from the corner's lower left to cause to top segment to move counterclockwise. That large external force came right from inside the building, and is also driving those multiple squibs.
Figure 6. Photo 0.5 second later, in which a new set of squibs have ejected out on 2 different floors. With those squibs the top tower makes a major turn counterclockwise, indicating it acquires a significant piece of angular momentum. The top segment has moved down further, indicating further disintegration at the bottom, and the gray cloud, which fed by the disintegration, has grown more.
Figure 7 shows the progression 0.25 seconds later than Figure 6, in which the fronts some of the squibs have moved close to 40 feet away from the building. These are moving at nearly 150 ft/sec (100 mph), and the source of the squibs is also the source of the large angular momentum, as well as the energy and linear momentum, that it imparts to the top 34-floor segment.
Figure 7. Collapse 0.25 seconds later, in which both the white squib clouds and the gray clouds have expanded.
The top 34-floor segment is disintegrating at its bottom, and the only feasible source of that process is explosive energy. In fact, the sources observed for the forces that provide the large amounts of linear and angular momentum and energy to the top 34 floors, are the 2 separate sets of explosions that were explicitly seen producing separate white squibs. Each of these are sets of explosions because the appearance of multiple squibs at once implies there are multiple sources each time explosions occur. Note there may have also been other explosions inside the lower floors of the top segment to produce its disintegration along with the rapidly expanding gray clouds. The white clouds created by the squib ejections are pulverized concrete below the plane impact areas, whereas gray clouds arise from pulverized concrete from the areas set afire by the plane impact, in which the pulverized concrete is mixed with black carbon from the fires. The rapid expansion of the gray clouds and rapid disintegration and collapse of the top section suggest that there may have also been these other explosions which produced them.
Note in addition that nothing has happened at this point to cause the lower segment of the tower to fall, except possibly for material damaged by the explosions at its top. The fate of that top 34-floor segment of the South Tower still does not explain why the bottom 76 floors collapsed. The 2 explosions at the top of this lower segment pulverized some concrete at the top of this segment, but that would not substantially affect the structure below that. Where did the massive amounts of energy come from that were needed to cause the complete collapse of the intact parts of the South Tower below the top 33 floors set ablaze from the plane impacts, a segment that experienced very little damage from the plane crash?
In Figure 8, we show a view of the collapse of the South Tower from another perspective 1 second later. The whiter clouds are from the pulverized concrete from explosions which produced the 2 sets of squibs coming out below the upper segment, whereas the gray clouds are from pulverized concrete at the lower portion of the top segment, emitted from disintegration of the lower part of the top segment. The top segment appears ready to fall to one side of the tower, but it never actually does! As Steven Jones states, after pointing out the sudden acquisition of angular momentum and subsequent toppling of the top segment of the South Tower "But then ... this block turned mostly to powder in mid-air! How can we understand this strange behavior, without explosives?" Without explosives it would be strange behavior indeed. The whole top segment disintegrates into gray clouds of dust and as well as many pieces of larger metallic debris, like the larger debris falling in Figure 8. These were clearly produced by explosions. In his paper Jones provides evidence that these explosions were caused by pre-positioned cutter-charges.
Figure 8. Another perspective on the collapse 1 second later, which shows the 2 different sets of dust clouds, as well as many metal beams flying away from the tower, mostly falling from the disintegration of the top segment in regions roughly on the other side of the gray clouds. Note that this perspective shows that as result of the rotation change in angular momentum at the base of the top segment shown in Figure 6, the fall velocity is now toward the southeast. However, the top of that top segment that is visible is located primarily south rather than east of the base segment , since the eastward component of the movement in Figures 6-8 has basically compensated for the westward component of the movement in Figures 2-5.
Figures 1 thru 8 show strong evidence for explosions through the
(1) an explosion imparts substantial energy and momentum to the top 34 floors of the South Tower, pushing them up several feet at the southeast corner, and creating a 9° leaning of the tower, then
(2) it starts progressively disintegrating of the lower part of that top 34-floor segment, causing the top of the tower to fall rapidly, then
(3) a second explosion imparts substantial angular momentum to the top segment, pushing the base westward and toppling its fall to toward the east, and the progressive disintegration of the top segment continues, while
(4) the lower 76-floor segment remains stable and intact, except for the white clouds at the top which indicate pulverization of concrete at its top.
Figure 8 by itself might suggest that the top segment of the tower strongly leans to the point that it would topple off the side. However, it does not, but it actually disappears into gray clouds and metal debris. Blasts that started the initial toppling of that top segment subsequently continued to dismantle it from the bottom up, so that it soon disappeared from bottom to top.
To produce crack like that observed in Figure 4, more force must be supplied upwards to the top segment of the tower from the explosive force below it than the force of gravity that is pulling it downward, in order to produce a net force to push it up. We can write the simple force equation for that as:
(3) ma = Fexup - mg
where Fexup is the portion of the explosive force that is pushing the top segment up, a is its acceleration rate upward, m is the mass of the effective portion of the top segment that is moving up, and g is the gravitational acceleration (32 ft/s2 or 9.8 m/s2). From Figures 4a and 4b the crack has opened roughly 5 feet (1/40th of the width of the tower) in 0.25 sec, which assuming constant acceleration throughout that short time and using distance s=0.5at2, corresponds to a=160 ft/s2 (49 m/s2) at during this 0.25 second (where it is at its maximum value). The full tower has a mass of about 4.5 X 108 kg (the same mass as 990 million pounds of weight), and the top segment is 0.3 of that. It is hard to judge how much of the mass is lifted up by the explosions, but let us assume the conservative value about 1/20 of the top segment, or 0.017 of the mass of the full tower. Then the upward component of the force out at this edge of the tower at this time is:
(4) Fexup ~ 8 X 107 N
With the upward force component applied to the top segment out at the edge of 80 million Newtons means that well inside the buiding these explosions are creating powerful forces. Thus since Figure 4 shows this upward component of the explosive force produces a raise up of the southeast corner (label lup) a little over 5 feet, we can estimate the net upward energy imparted to the top segment by that explosive force as:
(5) Eexp = Fexp lup ~ 4 X 108 J
Thus nearly half a billion Joules of energy (equivalent to about 0.1 ton of TNT, or about 20 sticks of dynamite) is imparted to the tower in a directly upward direction out near the tower's edge, and this is expected to only be a fraction of the energy causing the tower leaning observed in Figure 3. This means the power applied directly upward on the top segment out at this edge during this short time is over 109 W, or over a million kilowatts. A huge explosive force is needed to produce this large amount of power. But this is only a small fraction of the total power produced by the blasts well insider the tower at the floors just below the top segment, since not only have we made conservative estimates with direct observations one just one component of the force at the edge of the tower, but also much of the power goes into other channels such as breaking the walls, pulverizing the concrete and producing the squibs, etc. Clearly tremendous explosive forces are producing this power, and Figure 5 shows it then also starts disintegrating the top segment from the bottom up. This is from just the first set of blasts.
The disintegration of the top segment of the South Tower from its bottom take much more energy than it did to raise it upward at the corner. The first set of blasts provided energy not only for the upward movement, but also for the start of that disintegration. The second set of blasts shown in Figure 6 provides more energy both for significantly rotating the top segment and for continuing that disintegration, in which concrete is pulverized to produce the gray smoke. Hoffman  estimated that the energy needed just for pulverizing the concrete of the North Tower to be 135,000 kilowatt-hours (5 X 1011 J which is equivalent to over 100 tons of TNT), so if the same approximation is used for the South Tower, then 1.5 X 1011 J (42,000 kilowatt-hours or equivalently 30 tons of TNT) of that comes from top-segment disintegration. Tremendous explosive blasts produced that pulverization, disintegrating that top segment in a few short seconds.
These calculations show there was substantial power in these 2 blasts. Note that much more energy and power will be required to cause the breakup and collapse of the rest of the South Tower -- the lower 76-floor segment -- and smatter it into dust clouds, both processes of which were observed to occur. Analysis of the South Tower's energy of fall shows it requires substantially more than the total potential energy from gravity of the tower, just as the detailed analysis carried out for the North Tower showed substantially more energy used than in the gravitational potential energy available.
In Figure 9, 1 second after Figure 7, that last photograph is shown in which the top of the tower is still visible. It is falling below the bottom smoke line of the North Tower, which indicates over 16 stories of that top segment have disintegrated at the bottom of the segment and disappeared into the clouds expanding out. Well over 50% the top segment has collapsed, while the lower segment it just starting to collapse at its very top. This strongly contrasts with the "crush-down/crush-up model" for gravitational collapse of the Twin Towers that has been proposed, in which the lower segment "crushes-down" due to gravity first, then the top section "crushes up" after the lower segment hits bottom.
Figure 9. 1 second after Figure 7, in which the top of the South Tower is falling below the bottom smoke line of the North Tower. Over 50% of the top segment has collapsed and disintegrated at the bottom of that top segment, with the pulverized concrete coming out in the expanding gray clouds. The lower segment it just starting to collapse at its very top, with pulverized concrete going into the white clouds. 
These calculations and the evidence from that collapse prove clearly that plane fires and gravitational collapse cannot begin to account for the South Tower collapse as FEMA and NIST studies claim. Gravitational forces clearly cannot produce the power either that pushes the top segment upward or that produces the pulverization of all the concrete of the top segment, and it is inadequate to produce the collapse of the lower segment of the tower. Similarly, the "crush-down"/"crush-up" model proposed for gravitational collapse of the World Trade Center is wrong for the South Tower.
The fact that the source of power both for the squibs and the upwards
movement and eventual disintegration comes from inside the tower shows that
model also cannot be correct for the South Tower. Any models using
nuclear explosions are similarly quite inconsistent with the evidence
as these power levels are a few orders of magnitude lower than what a
nuclear blast would create.
The data is only consistent with conventional explosive forces inside the
tower causing the initiation and process of the collapse.
 The collapse sequence of the South Tower from which the photos in Figures 1-3, 5-7 and 9 were taken was filmed by NBC, and the crack-development sequence in Figure 4 was filmed by SCT-1 of Spiegel-TV. Full videos of these sequences were put online in the 9-11 Research Project, which posted several videos of portions of the collapse of the South Tower at: http://911research.wtc7.net/wtc/evidence/videos/index.html
 Stephen Jones, "Why Indeed Did the World Trade Center Buildings Completely Collapse?" Journal of 911 Studies, Volume 3, September, 2006, linked from journal index posted at http://www.Journalof911Studies.com
 Posted by the 9-11 Research Project at http://911research.wtc7.net/talks/towers/docs/site1101o.jpg
 Jim Hoffman, "The North Tower's Dust Cloud: Analysis of Energy Requirements for the Expansion of the Dust Cloud Following the Collapse of 1 World Trade Center, October 16, 2003, posted at http://911research.wtc7.net/papers/dustvolume/index.html
 Zdenek Bazant and Mathieu Verdure, "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions," Journal of Engineering Mechanics, Vol. 133, 308, March, 2007.
 Judy Wood and Morgan Reynolds, "Star Wars Beam Weapon" posted at http://www.drjudywood.com/articles/DEW/StarWarsBeam1.html