In the blog post Downward Acceleration of the North Tower, by David Chandler, AE911Truth, Mr. Chandler deduces from the application of Newton's third law (For a force there is always an equal and opposite reaction) to the visual evidence of the North Tower collapse, the force being applied by the falling block of floors to the bottom block was less that the force exerted by the top block at rest, and so logically no collapse should have occurred.

What follows is another way to apply Newton's third law to the problem, perhaps even easier to understand, and it yields the same result: Gravitational collapse theories are impossible.

NIST explains that the falling block, having acquired energy of motion as the support of the floor between it and the bottom block gave way, then had such a huge amount of energy that when it collided with the top floor of the bottom block, that top floor did not stand a chance against such a force that it was never designed to withstand. This floor would therefore give way without impeding the falling mass in any meaningful way, and so become added mass to the falling mass, thus increasing the energy even more, and in this way the falling block, ever increasing in mass and energy, would inevitably smash its way through the entire bottom block of the building.

There have been many very good arguments put forward to show that this line of reasoning is, to be charitable, less than adequate to explain what was observed on that September morning in New York. Unfortunately, many of these arguments very quickly involve math and physics that may make the average person's eyes glaze over, however valid they may be.

So let's keep it simple. Let's use an iron-clad law of physics and apply it to the collapse scenario to prove that the gravitational collapse theory is a theory that depends upon physical impossibility, and should therefore be immediately discarded.

The following is Newton's third law of motion, formulated over 300 years ago:

Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.

For a force there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.

Let's see how Newton's third law applies to some real world situations in order to clearly understand what it is saying.

A rocket engine initiates a force in the direction opposite to the direction of travel. It is the equal and opposite force that actually propels the rocket forward.

A Newton's cradle* pendulum is set in motion, and it strikes a set of stationary pendulums. Upon contact, the force of the moving pendulum is sent into the set of stationary pendulums. Most of the force is transmitted quickly through the set to the end pendulum, resulting in the motion of that pendulum.

And what happens to the pendulum that was initially put into motion to strike the others? Upon contact, it received an equal and opposite force to the one it sent into the set of pendulums. The end result is that it stops in its tracks, as the force it imparts to the set of pendulums is met with the equal and opposite force that is a result of the collision.

Billiard ball collisions also provide clear illustrative cases of Newton's third law.

So what happens when we apply Newton's third law to the Twin Towers' collapses?

Reviewing, here is the NIST hypothesis:

10. A falling block with a huge amount of energy (due to its motion) strikes top floor of bottom block.
20. This impacted floor is overwhelmed and gives way.
30. This collapsed topmost floor becomes part of falling mass, increasing both the mass and energy of falling mass.
40. This more massive, higher energy-containing block strikes the next floor.
50. Repeat steps 20 through 40 until the falling block reaches the ground.
60. The building is demolished.

As implausible as this scenario may be in the real world, let's grant NIST all of the above, but let's apply Newton's third law as well, which must have been in force on that day:

10. A falling block with a huge amount of energy (due to its motion) strikes top floor of bottom block.
20. This impacted floor is overwhelmed and gives way.
25. An equal and opposite force is exerted upon the bottom floor of the falling block in a manner identical to the way the force is exerted upon the top floor of the stationary bottom block. The bottom floor of the falling block is therefore also overwhelmed, as it is essentially identical to the topmost floor of the bottom block, and so collapses.
30 This collapsed topmost floor becomes part of falling mass, increasing both the mass and energy of falling mass.
40. This more massive, higher energy-containing block strikes the next floor.
50. Repeat steps 20 through 40 until the top block, which has fewer floors than the bottom block as is losing floors at the same rate, no longer exists.
60. The collapse stops.

For those who would argue that it doesn't matter if the top block becomes pulverized or not because the mass and energy remain the same in either case, I would contend that this is certainly faulty reasoning. The cases are not identical at all.

For example:

Consider a massive, 1,000-pound wooden battering ram that breaks down a wooden door. Now break the battering ram into a thousand 1-pound pieces and apply these disparate masses simultaneously to the door with the same force. What happens to the door?

Consider a 16-pound urethane bowling ball dropped upon a pane of 1/4-inch thick glass suspended above the ground. It smashes its way through with hardly a pause. Now break the bowling ball up into a thousand little urethane spheres of about 1/4 ounces each and drop the aggregate on the glass pane. Do you expect the same result?

The physics governing a solid mass is much different that the physics governing a collection of smaller masses equal to the solid mass.

And, of course, any objective viewing of the Twin Tower collapses leads to the inescapable conclusion that during the collapses much or most of the pulverized debris was being forcefully ejected out beyond the building footprints, and so this mass was in fact not contributing to the destruction of whatever floors remained standing below.

I would generalize this case by proposing the theory that no homogeneously constructed object, broken into two parts with unequal masses, can have the larger mass demolished by the smaller mass simply by dropping the smaller onto the larger, no matter how high up one decides to elevate the smaller mass before letting it fall upon the larger.
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*Most popularly seen as the executive desktop toy with five steel spheres connected to a framework by two strings each. See http://en.wikipedia.org/wiki/Newton%27s_cradle