You’re the one who brought up the air-in-a-bottle, not I.
OK then, let’s regroup. Let me repeat what I think is your argument:
- Spaghetti is sucked into your mouth for the same reason as tennis balls will force a rug under a door.
- That is, in the area near your lip, particles hitting the spaghetti first and then the lip have more energy than particles hitting the lip first and then the spaghetti.
- The additional energy is transferred to the spaghetti as a force pointing inwards toward the mouth.
- Normally, this force is balanced by the same thing happening inside the mouth, resulting in a cancelled force.
- When you suck, the reduced pressure decreases the force in the mouth, and the spaghetti strand is pushed inward.
- There must be a reason “particles hitting the spaghetti first and then the lip have more energy than particles hitting the lip first and then the spaghetti.” You’re saying it’s because the lip is cooler than the spaghetti.
Now, if all that’s true, then when the spaghetti is cooler than the lip, everything is reversed: particles hitting the lip first have more energy, and the force is pointing outwards, away from the lip. In this circumstance, when you decrease the pressure in your mouth, the unbalanced force will cause the spaghetti to move away from your lip. Or, more siply: When you suck on a piece of cold spaghetti, it will shooot out of your mouth.
Which, to be clear, I don’t believe actually happens. So there’s an error in your chain of logic, which is fundamentally back at the beginning. Air pressure acts perpendicular to the surface everywhere; there is no tangential pressure component in the way you envision.
Correct.
Right, I did. Now let’s look at a general air-in-a-bottle problem. Suppose the environment of the bottle is NOT at absolute zero. What do you suppose will happen? Why does the specific case where the environment of the bottle IS already at absolute zero lend more insight to the subject of this thread? Or did you have another reason for bringing that up? (I admit exploring limiting case is inherently useful and interesting.)
You’ve got me right up to point 6. The lip and the spaghetti are both “cooler” (less energetic) than the air. The air molecule is hitting two cool surfaces in quick succession. I’m saying the surface it hits first, it hits hardest.
Because there aren’t any “fresh” air molecules coming out of the lips (or the spaghetti), the first, hardest hits at the lip-spaghetti boundary are all inbound. The lips aren’t sucked in because they’ve got teeth behind them. So the spaghetti is sucked instead.
In which case your conclusion is that air molecules slow down after a collision, even when they’re colliding with things (like your lip and the spaghetti) that are warmer than the ambient temperature of the air.
So, to return to an earlier post: if I seal up a tin can (and, moreover, even if I slightly heat the outside of the can up to skin temperature) the air inside the tin can will lose energy, and the pressure will decrease.
Which, again, I don’t believe actually happens. So there’s an error in your chain of logic, which is fundamentally back at the beginning. Air pressure acts perpendicular to the surface everywhere; there is no tangential pressure component in the way you envision.
I knew I’d regret bringing “warm” and “cool” into this. A gas molecule is “cooled” (loses energy) when it hits something solid, even though the solid object might have a higher temperature. At a given temperature, gas molecules have much more energy than molecules of a liquid or solid.
No. As you heat the can, air molecules striking the inside of the can lose less energy than they did before, so the net energy rises. But each air molecule still loses energy when it strikes a solid.
Keep in mind that the energy lost by air molecules striking the inside of the can is normally balanced by air molecules striking the outside. Pressure change results from upsetting that balance. The temperature of the can itself is relevant only if it affects one side more than the other.
You keep saying that. Is it really a law of nature? So many of the rules we’re taught are generalizations. I believe what we have here is (literally) a corner case. I can accept the generalization for sucking solids and liquids, but for a semisolid it just doesn’t make sense.
Which is not to say I’ll stick to my vision if someone offers a better one. So far, I understand my vision, and I don’t understand any other.
So ignore temperature for now. You’re still arguing that air molecules lose energy every time they hit a solid. So let me repeat what I asked before: what happens over time in a closed container, as air molecules get slower and slower? You’re arguing that the molecules must eventually slow to a stop.
Which, again, I don’t believe actually happens.
Why not? If there aren’t any air molecules striking the outside, or any other source of energy to excite the air molecules inside, I see no way to keep them from slowing to a stop. Can’t you see them, lining the can in a pathetic little Heisenbergian cloud, each of them lying vaguely in the spot where it gave up its last little quantum thump?
Of course, that won’t ever happen in the realm of normal experience, because there’s always a source of outside energy.
Just out of curiosity, what do YOU believe is making the spaghetti go into the mouth?
But you said “each air molecule still loses energy when it strikes a solid.” If that’s true, it doesn’t matter if there’s a source of outside energy, because that energy is never transferred to the molecules inside.
So, one more time: since you think each air molecule still loses energy when it strikes a solid, what happens over time in a closed container, as air molecules keep losing energy? You’re arguing that the molecules must eventually slow to a stop, regardless of any outside energy input.
Which, again, I don’t believe actually happens.
Air pressure differential, ultimately, transferred through internal stresses in the spaghetti. I say more back in post #5 (which was the original post mwbrooks objected to), but I would be happy to expand on it if you like.
You might have me there. I’m not sure. Here’s what I think happens: the energy lost when an air molecule strikes a solid is momentarily gained by the solid molecule. But molecules in a solid can only contain so much energy because they’re locked in, and can’t accelerate easily. So the solid molecule re-emits the energy as photons, which either shoot off into space or strike other molecules. This happens throughout and on either side of the solid. Photons that strike air molecules are easily absorbed and translate into more speed. So energy can be transmitted through a solid even though the solid can’t contain as much energy as a gas can.
You seem to think I’m saying the energy lost by an air molecule disappears forever. I didn’t mean that. It is converted from velocity into something else. Once again I regret mentioning heat. My mistake was to forget that the heat of an object can be transferred back and forth between excitation and radiation. I’m afraid once again I’m out of my depth here, or I could have made my case much clearer. Sorry.
"The lean man gazed at me. ‘You realize, of course, what this means?’ he said.
"‘But I don’t understand, Professor Allors,’ I replied, thunderstruck. ‘You say that the theory of Newton regarding Gravity is false. How can this be? Why, if it be so, then, only to take the most obvious of examples, how is it that objects such as this cigar fall?’ Suiting my action to my words, I took a Trichonopoly from my breast pocket with my right hand and let it drop into my left. You see? How is this to be explained without Gravity?’
"He fixed me with his steely-gray eyes. ‘The answer is simple, my dear young friend. Your cigar, like all objects, falls because it has weight. Be but this conceded, and you can clearly see that your imaginary difficulty disappears at once.’
“‘Most amazing, sir!’ I replied. ‘It is all so simple, when you explain it thus.’”
-- Herbert Ryder Doyle: *The Emperor of Venus* (1908)You’re being abstruse, John.
Well, but heat and temperature are part and parcel of the way things work. If molecules lose energy when they bounce off a surface, they’re losing temperature. That would mean that, near any surface, there’s a constant stream of cold air being created by the collision with the solid surface. And, according to your explanation, this stream of cold air exists regardless of the temperature of the solid object.
Which, again, I don’t believe actually happens.
And, I suppose I thould have done this before: Gas pressure explanation from NASA.
He is NOT being abstruse. He’s telling you why he thinks your explanation stinks. 
Not cold, cooler. If it helps any, be reassured that the cooled molecules are reheated almost instantly by striking photons or other air molecules.
Ask yourself what you seem to be asserting: That molecules can impart force on a surface without losing any energy at all. If that were true, air pressure wouldn’t exist.
Note also that the force I’m talking about is tangential to the surface, as you demand. The tangent of an angle divides the angle. Our essential disagreement appears to be that I believe this tangential force acts over a small but finite area. You appear to believe the tangential force in a corner applies to an infinitely small area, and therefore doesn’t exist.
Excellent. I’m sure you read the sentence, “The pressure acts perpendicular (normal) to the wall; the tangential (shear) component of the force is related to the viscosity of the gas.” Your source asserts that there is a shear force imparted by air molecules!
In NASA’s case, the shear force is interesting only in how it produces drag. Aerodynamic drag is of course not an issue for noodles, unless you’re a noisy eater. However, the drag case is akin to ours because it involves a shear force exerted on a surface by air molecules moving more energetically in one direction than in the opposite direction. You seem to have asserted that this can’t happen at all. Did I misunderstand?
OOOPS! Replace “tangential” with “normal” or “perpendicular” in that. I just woke up.
…in which case your description of events is almost exactly like mine, and I’ll rephrase my original question: How wide to you think the boundary is on the spaghetti where the “non-reheated” molecules strike?
No, you’re fundamentally misunderstanding what causes pressure. Pressure is a transfer of momentum, which is not the same thing as energy. Force can be exerted without the loss of energy. Wikipedia actually has a fairly decent article on molecular motion and pressure (yeah, I know; Wikipedia. Still, it’s not bad.)
Shear force is caused by bulk fluid flow (or, rather a difference in velocity between the solid and the surrounding fluid). That’s what they mean by “related to the viscosity of the gas.” If you want to continue down this path, you would be arguing that there exists a substantial wind that literally blows the noodle into your mouth. Which, again, I don’t believe exists.
I infer he didn’t understand my explanation. That’s too bad.
Your explanation wasn’t an explanation. After going to a great deal of effort to dismiss two centuries of science on the subject of air pressure, you cannot wave around the words “air pressure” as though, according to you, they still explain anything. You’ve sawn off the branch you’re sitting on, which is amusing, but hardly illuminating.
If you have an alternative to the kinetic theory of gases, give it.