The system is the mobile part of the building, which is not closed in that new layers become mobile and are added to the moving mass.
The moving part gains momentum by the addition of more mass by each smashed and now mobile layer releasing its PE as KE, so the energy of the moving bit increases. Note momemtumm is p=mv. The mass of the moving bit increases, it also accelerates, so the v bit goes up too.
It is important only to invoke science if you understand it.

You can say that again.

Precisely!!! I’d need a thirty ton tea bag to stand any chance of hammering in a nail. Why?

Indeed; but would it require a controlled explosion to make 000s of unconnected blocks collapse into a pile? Would the same thing happen if all the blocks were glued to their adjacent blocks?

If we took the top glass from a stack of pint glasss and dropped it back in from, say, 50cm*, will it a) fall to earth, breaking all the other glasses before breaking itself, or b) break some glasses and possibly itself before coming to a halt? If we took a steel-framed tower construction, damaged fewer than 20% of supporting columns across less than 5% of its height, would you expect the portion above the damage to fall symmetrically vertical and destroy the entire structure below it before then destroying itself?

*relatively, way further than the tops of the towers descended

No need to be a dick, maxtorque. And I didn’t say that anything was blown up by the US Government; Congress would never allow it Shirley.

It’s not in any way a direct comparison, but I’ll go along.

Yes, in your example the first floor would probably be crushed. Also, the third floor would probably exhibit considerable damage. Damage (distortion) would diminish rapidly from this point, though it’s possible that the tower would actually topple over.

I chose this for the soundtrack; other videos are available:

Why would it have taken such a massive team? And so long?

Is that a question? Or an answer? It doesn’t really explain anything.

Hmmm. Makes you think…

Well the mass has increased and it has been accelerated by gravity. so both v and m are increased, so yes.

Ok, so F=ma stops working, carry on.

Not trolling, just keeping the science on track and applying a slightly critical eye to some of the statements being made.

Also, “because they are big” is a bit of a crap answer really isn’t it?

Sure, it’s a problem there is some elasticity in the building, more the higher you are. But I think for now we could consider the force at the interface between the falling bit and the bit underneath. So we have the force crushing the floor below and at the same time slowing the mass above, or at least reducing its momentum.

A simple experiment.

Take a raw egg and place in an egg-cup. Now take a 1Kg weight such as that from a set of old fashioned weighing scales and balance it on top of the egg. It will be supported no problem. Now lift the weight a couple of cm and drop it. The egg will crack and break into pieces.

That’s the difference between a static and dynamic system.

The lower 85% of the tower was only designed to hold up the mass of top bit “statically”. There’s some redundancy in the design to allow for excess load but that’s usually in the order of a few percent.

It’s highly unlikely either top section of the towers would have fallen over. The towers were approximately 70mx70m in cross section so the centre of mass would have had to move at least 35 metres to lie outside the building walls. Each floor of the towers was just under 4m in height. The north tower was hit between the 93rd and 99th floor so there’d only be 11 to 17 floors of material, i.e. a maximum of 68 metres meaning its CofG would be around 35m above the impact. So for it to fall over that point would have to move 35m horizontally. Similar quick calc for the south tower gives a CofG for the section above the impact zone of 60m.

Look at video of the tops of the towers as they begin to fall, they wobble by a few degrees but nowhere near the amount needed for them to topple over. The path of least resistance for the falling mass was through the structure below.

I’m not, I want to get to resistance in a minute, will you answer my question?

You asked why it would take a long time with lots of people. I suggested it was because it was big. Big buildings take lots of time and lots of people to rig for demolition.

Unless you know a quick way. Like fly a plane into it…

Have you seen the preparation required for demolition by controlled explosion?

Except for most skyscrapers, or even large buildings, they have far more in common with the eggshell example that keeps cropping up.

No, it was 80 something floors of steel spanning structures that got thinner the higher up it went, no structural concrete at all.

Twin towers were mostly empty space

I already told you, your example is not comparable. Why are you going to add resistance later? Just put it where it’s meant to be. And why not stick to an example where we can observe hypothesis (or lack of)? I’m busy, I’ve a load of work to do and, to be honest, considering pointless examples of incomplete equations is pretty much bottom of my list of priorities. If I’m missing something, you’re going to have to less obscure.

Its a model. Resistance is my next question. This is what we call a discussion. It’s a shame you want to withdraw now.
I wanted to ask you how the resistance effects the very first failure. There is an interesting discussion to be had about that. But we need to establish a few agreed facts first.so please answer the question?