Wednesday, March 10, 2010

Newton's Laws

Dear Billy,

(Cartoon is a classic from xkcd.)

I have addressed your points - Everything you say is against Newtons Laws - an aluminium plane cannot do what you are claiming.

All videos of 911 show the planes melt into the buildings, with the builings sealing themselves up after the plane has entered, then exploding inside after coming to a sudden halt, then the building is supposed to have weakened and collapsed because of the fires from the plane fuel which burns at 816 Degrees Cent (in an enclosed space even though you see the fuel burn up in seconds on the videos) when in fact steel melts at 1482 Degrees Cent.

in lieu of a longer post, which will have to wait until I finish work today, I am posting "Newton's Laws" - I assume you meant 'Newton's Laws of Motion" - I can't see what gravity, cooling or viscosity have to do with this.) Which of these states that an aluminium (they're not, really, but it probably doesn't make that much difference) aircraft will bounce off (or be completely stopped outside of) a modern building?

1.  Law of Inertia. Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.
2. The change of momentum of a body is proportional to the impulse impressed on the body, and happens along the straight line on which that impulse is impressed.
3. Law of Reciprocal Action: To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

Now, when a Boeing 767 hits the glass and concrete facade of a steel-frame building, it will slow down. Rapidly. At 300knots there is nearly 20 million N.s involved in that impact. Moving at roughly 150 m/s, and at under 50m long, you are going to get a lot of force.  Basically, Law 1 says that, unless you have a force, the plane will keep on moving.  That force comes, mostly, from the structural materials of the building.  Law 2 allows you to calculate or estimate the forces involved and you then start to have to worry about sheer and other stresses (oh, and the integrity of welds and effects of corrosion and all sorts of clever materials effects).  Law 3 says that the necessary force on the plane is matched by an equal and opposite force on the building.  Please note here that you maintain that the plane(s) could not have gone in to the building- that they would have been stopped by the concrete surrounds of the windows.

Glass and the thin concrete in the facade simply do not have the shear toughness to take this and crack or crumble, failing to provide enough force to stop the plane,  which carries on into the building. Slowing down quite rapidly and falling apart (because aircraft aren't tough enough to take this sort of thing either) - although some of the bits, especially the engines, will keep going for longer being both more dense and more sturdy. At some point (not many seconds later), a large plane, like a 767, will hit the frame and either shatter that (causing a fairly immediate collapse) or come (mostly) to rest. A light plane (Piper Dakota, for example) will wear off its velocity against other objects - desks, computers, people, filing cabinets, whatever - and may not reach the main frame - especially if the floors are quite strong.

I will note here that as kinetic energy is proportional to the square of the velocity and momentum only linearly, then there will now be a lot of "spare" energy about.  Under the first law of thermodynamics, this can't just wander off and go for a coffee, so has to hang around.  Most of it will be as heat energy.  Which will have a bad effect on all of the fuel now inside the building ...

A slightly heavier (than the Piper) but much less tough (either then the Piper or a 767 - aerospace technology has improved dramatically since then) plane (a 10 tonne B-52 B-25 Liberator bomber, without bombs) hit the Empire State Building in 1945.    No direct energy weapons then.  Eye witness accounts, from inside the 79th floor, have the plane exploding inside the building ...


Billy Carlin said...

Deary Deary Me! Glass and thin concrete on the facade?

1 inch thick steel girders almost as wide as the narrow windows in between covered with aluminium cladding.

Pictures of the real facade and the fake planes melting impossibly through the steel girders at Paisley Expressions.

Angry Walrus said...

Small point but it was a B25 Lib, not a (much more recent) B52.

Good article though. It's always worth mentioning that steel doesn't need to melt before it collapses. It weakens considerably at temperatures well below its melting temperature.

these guys show that even toughened steel bolts have less than 10% of their orginal strength at temperatures of 800 degrees C.

Tough to imagine a building would standup when the girders supporting it are only 10% as strong as they need to be...

Surreptitious Evil said...

AW - thanks for pointing out the typo. Obviously, apart from the time machine necessary to have a BUFF around in 1945, a B52-H has a loaded weight of around 120 tonnes - a roughly equivalent mass to the 767-300.

As for the steel - yes, there is another post coming - the decay of sheer strength with temperature was going to be (and still will be) one of the points raised.

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