Elementary HS physics may not quite be up to the task of explaining a large building's collapse.

Absolutely, but it can tell you about the upper limits of the possible speed of collapse, which is significantly less than free-fall. One of the limiting factors is the conservation of momentum. Gravity is the only force that speeds up the fall.

Conservation of momentum holds only in systems in which there are no forces acting on the masses.

I think you're thinking "no external forces" - i.e. no forces from outside the system. If we start considering gravity to be some mysterious unknowable force, or that anything can happen to momentum without forces acting, then we'd better stop flying for a start.

The reason it's relevant is because following the official "pancake theory", each floor was stationary when hit by the floor above, and the momentum of that floor would have slowed the falling mass from above. Gravity does not somehow cancel this effect.

The velocity of a falling object is a factor not only of the force of gravity and its mass (which, I suppose...

The acceleration due to gravity does not vary according to mass, as Galileo Galilei worked out some time ago. Air resistance, which you mention, brings mass into the velocity equation. However, it would cause the descent to slow down. The issue here is that the descent was too fast.

The slowdown experienced as a result of crushing a lower floor would likely be well within the margin of error for any reasonable estimate.

Oh well, if you say its likely to be within a reasonable estimate, then that's all right then isn't it. QED. Nicely argued. Now for the extra points, can you show your workings?

nerdnam chimes in with:

Think of a bullet going through a leaf.

Think again. So what are you saying here? - that the upper floors of the buildings were super heavy but the lower floors were much lighter? Paper? Novel building design.

It's impossible to prove anything true, but it only takes one counter-example to prove something false. If you need a counter-example with simpler and more intuitive physics, then the pools of red-hot molten metal at the bottom of the towers, weeks after 9-11, is all you need to know. Worry about the physics of falling buildings after you've debunked that mystery.

--abc

Gravity, alas, was in play normally in New York that day. What was wrong from the moment WTC was designed on the cheap remember - that was its initial "selling point," were the floor "L" joints. It was a miracle they held their own floors for as long as they did, let alone others crashing on top. Remember too, the elevator columns were blown out, having never had reinforcement – so no internal support; just burned away ply board.

And Mr. Owen, do see "United 93" - well, see it, if you want to know about "where were the interceptors"? It's all there.

And all of us should have, by now, noticed we really don't have a military or any intelligence agencies capable of creating cleaverly designed alternatives to the truth, or more problematic recognizing cleaverly designed alternatives to the truth. Dream on.

The American government could not conspire to buy a ham sandwich.

Conspiracy requires:

1) competency

and

2) secrecy.

If a guy can't get a blowjob under the Oval office desk without some ugly old secretary finding out about it and causing him to be impeached, there are no holographic planes and controlled demolitions.

Come on. At least one person who designed the holograms or set the explosives (or his disgruntled ex-wife) would be ready to cash in on the book deal.

So Charles your contention is that because the government is stupid there are no secrets left. Because they are incompetent any conspiracy is impossible. I am glad we know everything now! All the secrets have been told. Wow. Nothing left, not even who killed Kennedy. Just because you think they are stupid.

This is a silly argument sorry. As for holograms, you are attacking the straw men thrown up by Patrick and his band of loyal skeptics. Even conspiracy theories have a mainstream, and holograms are not on for most of us either. You don't need holograms.

And who would come forward about their involvement in a plan like this? Why would they do that? What's their motivation? Who would they go to with the information? FOX? Patrick Smith?

The resistance to the collapse was in the floor joists. And by joists we mean the bolts which held the floors to the frame of the towers. When the upper stories of the towers collapsed onto the floor below, the floor joists on that floor could not bear the weight of the floors above. The floor was only built to hold its own weight, not ten or twenty floors. Thus the bolts broke and the floors above impacted the floor below.

The next floor was even less able to resist the weight of the floors above. If ten floors collapsed initially, then by the time the 20th floor was reached, there was now twice the weight impacting the joists, meaning the joists could only offer half the resistance of the bolts that originally broke. By the time the 40th floor was reached, the weight was doubled again and thus the resistance was cut in half again. This is a geometric progression.

Did I mention there were 110 floors? Yes, the upper mass did in fact end up slicing through the lower part pretty much just like a bullet through a leaf. Any resistance the joists had was completely overwhelmed by the pancaking of floors. Once the joists were ripped out of the frames, the frames were just sticks of wire and were dragged down with the falling mass. There was nothing that could resist the fall. Any variation from free fall would be minor and hard to detect.

The WTC towers were built lightly compared to the Empire State Building, which is of solid steel and concrete. Steel and concrete can only go so high without collapsing from its own weight, thus the ESB becomes progressively narrower so that it can be as tall as it is. But the WTC towers were just light frames with bolted on floors. This allowed them to reach the heights that they did and to be sheer sided as well. But it also allowed them to collapse in the way that they did, because the only thing really holding up the whole structure together and up was in fact the floor joists. Unlike the ESB, there were no solid columns holding up the building.

A good article explaining all of this in much better detail than I can:

http://vincentdunn.com/wtc.html

Note the part where he says the floors in modern buildings are only designed to hold the weight they are specified to hold. IOW, floors are no longer overdesigned to hold twice their weight as was done in the past. Now if this is true, then ten floors falling on one floor in the WTC towers would have exceeded the capacity of the floor by a factor of ten.