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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.
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Date: 2008-01-24 06:53 pm (UTC)no subject
Date: 2008-01-24 11:46 pm (UTC)Yes, you can suck spaghetti... Cooked spaghetti. The part of the strand inside your mouth expands, as it is no longer subjected to atmospheric pressure. However, this expansion is not a straight step: the expanding portion trapped between your lips forms a curving slope, with the sloped portion pressing outward against your lips, both radially and axially, with the axial force propelling the spagetti inwards into your mouth.
You can do this with soft and floppy compressible cooked spaghetti. The rigid uncooked strands do not expand significantly in your mouth, no matter how hard you suck, and the atmospheric pressure on the very end of the strand exerts a negligible force compared to the friction at your lips.
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Date: 2008-01-24 11:48 pm (UTC)It didn't occur to me the answer might be *that* wrong. I never have, but it sounded right to me. Hold on while I boil a strand of spaghetti.
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Date: 2008-01-24 11:53 pm (UTC)I expect you're right, and that sounds like it could be, but do you know that for sure, or it just seem the obvious reason to you?
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Date: 2008-01-25 12:41 am (UTC)no subject
Date: 2008-01-25 12:41 am (UTC)no subject
Date: 2008-01-25 12:42 am (UTC)no subject
Date: 2008-01-25 12:44 am (UTC)Googling for the physics of sucking spaghetti reveals no convincing references for my hypothesis.
no subject
Date: 2008-01-25 12:51 am (UTC)Now repeat the experiment with cold spagetti that's been left out in the air for half an hour: it's still floppy and compressible, but the surface is sticky. Try sucking that in! Manipulating it inwards with your lips is now easier, but still far less efficient than applying suction.
This does nothing to prove or disprove my theory of how the suction works.
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Date: 2008-01-25 01:08 am (UTC)That seems to support your theory (though not 100% conclusive).
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Date: 2008-01-25 01:09 am (UTC)The air pressure in that axis acts on those projected areas (you may have to do maths with angles to prove this to yourself), to give equal forces in both directions. In the spaghetti case, imagine the end just at the mouth. the air is not acting on a small area, so there is an overall force.
Once there is some spaghetti inside the mouth, you can do the calculation for both parts separately. You'll end up with a force on from each side, but the higher pressure outside means more force, so it wins.
---
But yeah, since it works with cooked spaghetti and not uncooked it must be the elasticity.
Solids will work by pressure differential alone for a large enough radius (e.g. pencil), because the cross sectional area (therefore force) increases faster than circumference (friction).
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Date: 2008-01-25 01:17 am (UTC)no subject
Date: 2008-01-25 05:48 am (UTC)no subject
Date: 2008-01-25 11:07 am (UTC)Well, of course. We still need some theory, but it was really easy to try, and I didn't have enough faith in our speculation to assume it was right. And I was right because the result really surprised me (though maybe not hairyears) :) I admit I felt a little silly boiling a single strand of spaghetti and then sucking it in and out repeatedly, but we all have to make sacrifices in the cause of science :)
So now we have evidence, and we have a theory that fits the evidence, and we just need to come up with some confirming experiments.
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Date: 2008-01-25 12:52 pm (UTC)no subject
Date: 2008-01-25 12:59 pm (UTC)no subject
Date: 2008-01-25 06:02 pm (UTC)Imagine doing your flat-projection-force calculation for all of the spaghetti except the first inch approximately horizontally sticking out from your mouth. The air pressure on that part is the same whether you're sucking or not, but it moves when the first inch is sucked into your mouth, so where does that force come from? Must it not be along the strand of spaghetti?
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Date: 2008-01-26 02:04 am (UTC)Say you had 1 inch sticking out North, a sharp bend, then a couple of inches going West. Ignore gravity (you are on SpaceShipTwo). It would be incorrect to state that the force is transmitted from the western end along the spaghetti, round the bend, then to the mouth.
This is hard to explain uninteractively. I need paper and a pencil to fight over, or maybe just hands to wave expressively.
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Date: 2008-01-26 02:39 pm (UTC)*confused* Was I saying that? I didn't think so, but then I was thinking aloud.
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Date: 2008-01-26 02:39 pm (UTC)Oh yes, very. Diagrams?
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Date: 2008-01-26 09:42 pm (UTC)Once the bit of spaghetti at the mouth is moving, the rest of it follows because it is being stretched. That force *is* along the axis.
The part I was trying to answer was "Why does it move at all?", or more specifically "Where does the push come from?".
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Date: 2008-01-27 12:08 am (UTC)Why does it move at all?
Ah, right. Of course. I think, originally I was saying the force *wasn't* propagated all the way along from the end. If anything (assuming we were in a case where a vacuum *did* suck spaghetti) I'd guess the air acting on the first inch of spaghetti, maybe along the axis of that from the bend. However, I was fairly sure that:
(a) If you drew it all out with air pressure arrows and integrated it a good answer would fall out, and
(b) I was not capable of doing this articulately.
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Date: 2008-01-29 01:35 am (UTC)Firstly, I don't know about cooked spaghetti, but a pen can be sucked into your mouth just fine, which is roughly equivalent. You know that enough force can be generated to raise that kind of mass or you couldn't drink through a straw. The amount of friction of cooked spaghetti seems to have been wildly overestimated.
Second, you know force along a rope can change direction using a pulley. It seems reasonable that a similar thing is happening here.
If you were to draw a diagram, you would see that the pressure difference is pulling the top of the spaghetti into the mouth, and the lower lip is acting as a pulley. I see no mystery.
no subject
Date: 2008-01-29 01:58 am (UTC)Not at all. LJ is strange in that it suggests chronological arrangements, but there's no reason you can't comment on an old post if you had something pertinent to say -- and a few days is barely old at all. I'm impressed at the amount of humour and science and maths produced here :)
> Friction
Wildly overestimated where? Pens and pencils can indeed be sucked. (I won't try right now as I went into work to get something on my way home, and am at that computer checking email before going home to straight to bed -- so there's a security camera on me :) But I tried it before to be sure, and you can.)
However, raw spaghetti seems unsuckable. Which suggests something about its mass/cross-section/friction ratios, although I don't know exactly what. And that cooked spaghetti seems (please someone confirm this) only suckable when you squeeze, suggests it is in fact naturally unsuckable.
I think I missed your point though, what were you saying.
> Pulley
I think I know what you mean, but I'm not quite sure. Can you expand?
That a normal sucking effect applies to the start of the strand of spaghetti, and that this then pulls the rest, like pulling on a rope pulling the rest in on a pulley?
I was saying *something* like that to Dave above. Did you see? However:
(1) It's not exactly a pulley on the lower lip, as cooked spaghetti normally isn't completely floppy -- it sticks out for a bit, *then* falls down, so the change in direction of the force is at a bend an inch from your lips.
(2) I'm not sure if how the force is transmitted is the key question, or only subsidiary to my suggested answer. I think the bit that really needs explaining is further back -- how air pressure forces (all inwards) turn into a sucking force on *any* of the spaghetti at all.
It's obvious that it *does* (although see next point), but for, eg. lift on a balloon you can draw a nice diagram of a balloon with arrows pointing down onto the top saying "force down" and arrows up below it saying "force up (slightly more due to lower in atmosphere)" and a big one saying "gravity" and it all cancels out and gives a little upwards force. You can do something similar for sucking a liquid. But what simple diagram is like that for spaghetti?
(3) As concluded above, the evidence provisionally seems to support hairyears' theory that it isn't really sucking at all, and so this is still interesting to know how it *would* act if you sucked, eg. a bendy pencil, but is *not* how you suck spaghetti.
However, I'm not sure if I got you, so what I said might not be relevant. Am I making any sense?
no subject
Date: 2008-01-29 07:02 pm (UTC)The friction is wildly overestimated when it is said to be 'obvious' that the pressure differential acting over an area the size of the cross-section of spaghetti would be much smaller than the frictional force between the spaghetti and your lips.
No, it falls more gradually. Also, you are likely to tip your head down to suck up spaghetti, causing a pressure force component in the vertical direction. The forces of pressure and gravity will be cancelled by a reaction force at the lip (draw a diagram if you like).
Think of a section of the spaghetti as a small plug between the lips - this will clearly be sucked in/forced into the mouth by the pressure difference. Adding bits onto the ends (where the pressures balance) isn't going to make any difference, the force is simply a pressure differential over the cross-section of the area of the spaghetti between the lips (where the forces don't balance), forcing the spaghetti in to the mouth.
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.
no subject
Date: 2008-01-31 02:08 pm (UTC)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)How does air pressure turn into a sucking force?
Date: 2008-01-31 02:28 pm (UTC)Firstly, the original question, "How does air pressure turn into a sucking force?"
I never understood pressure well. You would say, air pressure on the spaghetti makes the internal pressure within the spaghetti equal to the air pressure? And that then, through the lips, the difference in pressure is a force, pushing spaghetti in?
Does that apply to a solid pencil as well?
Re: How does air pressure turn into a sucking force?
Date: 2008-01-31 02:33 pm (UTC)no subject
Date: 2008-01-31 02:38 pm (UTC)Sorry, I'm slightly confused here, did you read the other comments? Quite likely you're entirely right, of course, "the reason you can suck spaghetti is the same as why you can suck anything else" seems a much more natural theory, and "cooking reduces the friction" and "you imagined that spaghetti had to be squeezed more" would be good explanations of what I observed.
(And what was that about someone saying something was "obvious"? I'm sorry if I or anyone sounded facile, I didn't think anyone *had* been blithely assuming anything about friction, just guessing, without certainty that they were righht...)
But your comment sounded confusing to me, it sounded like hairears had a plausible theory, and you came along and said "no, that's all wrong, its the obvious way," but without actually saying how you could tell the difference. But I don't think that's what you did mean, but I'm not sure what you did mean.
If you assume a normal sucking procedure applies to spaghetti, and the wedge theory is wrong, what experiment would show for sure -- we ought to be able to show it easily, oughtn't we?
Re: How does air pressure turn into a sucking force?
Date: 2008-01-31 10:58 pm (UTC)Take a rigid cylinder, and put it between your lips, and suck (not for real, although you can if you like). The force on a side is the pressure times the area (where the pressure is scalar, and the force and area vectors). The forces cancel out on opposite curved sides, but there is more force on the higher pressure side than the lower pressure side, pushing the cylinder into your mouth.
If the question is "why is the force related to the pressure", the answer goes something like, the pressure is related to the energy/momentum/that kind of thing of the molecules in the gas. The force is caused by transfer of momentum/energy from the molecules in the gas bouncing off the solid. The higher the pressure (more molecules or higher molecule velocity), the higher the force on the solid. Sorry for not being more specific, I never got on with thermofluid things.
The key thing to remember with spaghetti is that it doesn't know it's spaghetti, i.e. where you perceive its ends to be is irrelevant. The change in pressure is over the section in your mouth/the projected ends of that cylinder on the outside of the spaghetti.
It does apply to a pencil as well.
no subject
Date: 2008-01-31 11:12 pm (UTC)That's what I'm asserting. But I'm not a strand of spaghetti...
I did read the other comments, but they seemed based on the assumption that spaghetti being the same as everything else was wrong.
hairears asserted that:
"the atmospheric pressure on the very end of the strand exerts a negligible force compared to the friction at your lips."
I didn't mean to imply that anything said was facile, I'm just failing to express myself clearly due to posting on-my-way to things/otherwise in a hurry.
Hairears' theory suggested that the spagetti expanded inside the mouth, causing it to press against the inside of the mouth. This would not work, because there would be no force to press the spaghetti and the mouth together (I think, but I'll think about it). It mostly just seemed over-complicated, and likely to produce a smaller force (if it does work) than the direct sucking method, especially as spaghetti doesn't expand very much.
I would suggest that if you coat dry spaghetti in oil, it may be suck-up-able.
hairears asserted that: "the atmospheric pressure on the very end..."
Date: 2008-02-01 01:28 am (UTC)no subject
Date: 2008-02-01 08:35 pm (UTC)ROFL :)
no subject
Date: 2008-02-01 08:40 pm (UTC)Hm, yes. I have pesto.
*thinks* If difference in cooking is friction, coating in substance ought to mostly alleviate that? Though its hard to tell because it might be rubbed off in mouth. But if substances are the same in all coatings, suggests just a simple matter of friction after all. If not, suggests something more weird is going on. Maybe.
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.
no subject
Date: 2008-02-06 11:14 pm (UTC)Reason 2 is that the lower pressure inside causes the mouth cavity to contract. Take an inflated balloon and draw a circle on it, reduce the size of the balloon and the drawn circle will get smaller. Lips are pressing inwards.
I remain unconvinced that this is the main or deciding force.
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 :)
Re: How does air pressure turn into a sucking force?
Date: 2008-02-06 11:44 pm (UTC)(1) Your description of pressure acting on the ends sounds like my description I just made of air molecules, in that if the force is acting up from the bottom, it doesn't correctly predict that spaghetti will be sucked up rather than buckle?
(2) Contrariwise, if you consider pressure acting across a small plug in your lips, surely that implies variable internal pressure within the spaghetti, which we both thought there wasn't?