(no subject)
Jan. 24th, 2008 05:39 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
jackhttp://www.straightdope.com/classics/a4_184.html
Straight dope asks "How can you suck a strand of spaghetti?" The question being, sucking a liquid, you create a vacuum in your mouth, and it's generally enough to say the pressure within the liquid is less than that in the mouth, so a force acts on the liquid in the straw. Sucking a solid object (eg. pencil), you can say the air pressure cancels out all over, except down the length.
But spaghetti is floppy. The air pressure on the *end* of the strand can't be relevant, because pushing their wouldn't force it into the mouth.
The answer doesn't seem very satisfactory. I'm sure it's something like, air pressure generally acts all over the surface, perpendicular to it, and this cancels out all over[1]. Except on a line through the part of the strand through the lips. So there's a force on that part, propagated down the strand to the next bend (where it acts sideways to the strand).
But I can't really put that into words (or symbols). Can anyone else provide a simple, satisfying description?
[1] May be hard to show, either by common sense or integrals, but we know it *does* because the net air pressure on a strand of spaghetti in midair (neglecting variations with height) is zero everywhere.
Straight dope asks "How can you suck a strand of spaghetti?" The question being, sucking a liquid, you create a vacuum in your mouth, and it's generally enough to say the pressure within the liquid is less than that in the mouth, so a force acts on the liquid in the straw. Sucking a solid object (eg. pencil), you can say the air pressure cancels out all over, except down the length.
But spaghetti is floppy. The air pressure on the *end* of the strand can't be relevant, because pushing their wouldn't force it into the mouth.
The answer doesn't seem very satisfactory. I'm sure it's something like, air pressure generally acts all over the surface, perpendicular to it, and this cancels out all over[1]. Except on a line through the part of the strand through the lips. So there's a force on that part, propagated down the strand to the next bend (where it acts sideways to the strand).
But I can't really put that into words (or symbols). Can anyone else provide a simple, satisfying description?
[1] May be hard to show, either by common sense or integrals, but we know it *does* because the net air pressure on a strand of spaghetti in midair (neglecting variations with height) is zero everywhere.
just one more thing...
Date: 2008-01-30 01:39 pm (UTC)Real spaghetti will curve gradually down due to its bending stiffness. Forcing one bit horizontal between your lips will force the next bit to rotate closer to horizontal etc. so that it keeps roughly the same shape.
Re: just one more thing...
Date: 2008-01-31 02:17 pm (UTC)Sorry, that's what I meant, its just hard to describe without diagrams. I think what I was trying to say about pulleys was:
(1) I think we all agree that if there were someone inside your mouth pulling on the spaghetti, the whole strand would be pulled in.
(2) If a sucking force of whatever sort took the place of the little person, the same thing would happen, agreed.
(3) But my question wasn't "given that there's a sucking force, how does it propagate down the spaghetti" but "what is the fashion in which the sucking force arises in the first place?"
But I'm putting words to things as I go along, so I will never have been very clear.
I think its clear how that pulling action happens, and irrelevant to the most interesting questions (firstly, how air pressure creates a sucking force, and secondly, if that, or the wedge theory, applies to spaghetti) whether a pulley is a good analogy for that pulling.
Re: just one more thing...
Date: 2008-01-31 02:20 pm (UTC)Re: just one more thing...
Date: 2008-02-04 05:10 pm (UTC)The answer is (imo): because you are pressing on the sides as well, just as much as you are pressing on the end of the spaghetti outside your mouth. If the maximum resistive force of friction is less than the pressure force + gravity, then that is the only direction which the spaghetti will move in. More like squeezing toothpaste than pressing on the end.
Or: if you press on the end of spaghetti and it starts to buckle, the force now acts in a positive feedback way, to make it buckle more (because bending is easier than compression). If you are sucking spaghetti and it starts to buckle, the force-imbalanced piece of spaghetti will move closer to the lips, acting as a negative feedback.
So it is an interesting question, but I don't think that strange things are happening.
Re: just one more thing...
Date: 2008-02-06 10:41 pm (UTC)What you said about the buckled part being pulled back into place is true, but I'm not sure that there is ever any buckling.
Re: just one more thing...
Date: 2008-02-06 11:40 pm (UTC)I think I sort of see what you're saying. But... well, thought experiment. If you were sucking a pen directly upwards, I would have explained it in terms of molecular motion as "sideways cancels out, and air molecules hit the bottom harder/in more numbers than the top, so push the pen up more than gravity pulls it down." Obviously that's a certain level of abstraction, but I would have assumed it was a good description.
However, if you try a similar experiment with spaghetti, but then instead of lowering the pressure at the top, you increase the pressure, solely up into the bottom end of the strand, (for instance, by putting your flat hand below it and slowly raising) you find the spaghetti isn't lifted, but buckles[1].
But my original explanation would have predicted the same thing happens in the air pressure case and the hand case, so it's obviously not correct. But what *is* the correct explanation?
[1] I admit, I did not actually perform this experiment. Someone feel free to prove me wrong. I'm pretty damn sure though :)