[sub]I know it’s been discussed in the past here. But I feel the need for an update.[/sub]
If pressure variance causes objects to be pushed (blown) away from the higher pressure rather than pulled (sucked) toward the lower pressure, then how does sucking on a spaghetti strand make it go into your mouth?
Cecil has already covered this, but I don’t think agree with his conclusion. My guess is that it is a combination of higher pressure outside the mouth (minor factor) and one other thing.
The other thing is, IMO, the non-rigidity of the spaghetti. When a piece of spaghetti is sucked on, the portion inside the mouth swells slightly, due to the lower pressure, while the portion outside the mouth is at its normal width. This creates a ‘ramp’ of sorts, causing the spaghetti to ride down the ramp, sort of like the way a surfer rides a wave or the way a slowdown in traffic moves backwards up the flow of traffic.
| <--lips
spaghetti | |
V | /-------------------
===================== +
| \-------------------
|
outside mouth | inside mouth (low pressure)
(high pressure)
If I’m right, then the swell just inside the lip presses against the lip and forces the spaghetti inside. Supporting my theory is that the Spaghetti needs to be a) soft, and b) wet to really get a good velocity. The standard pressure gradient assists, since the center of the strand gets pushed into the area of lower pressure, adding to the swell.
[Edited by C K Dexter Haven on 12-27-2000 at 08:19 AM]
You realize that you’re an evil, evil man for resurrecting this discussion, don’t you? Well, I guess I’m abetting that evil streak of yours.
Anyway, for those that weren’t here for the first one: it was discussed forever in response to one of Cecil’s columns (in that forum). Here’s the column:
How does one suck in a piece of spaghetti? (14-Feb-1997)
And here are the three threads I found. The first one here is the long one, though it wasn’t first chronologically.
http://boards.straightdope.com/sdmb/showthread.php?threadid=582
http://boards.straightdope.com/sdmb/showthread.php?threadid=1507
http://boards.straightdope.com/sdmb/showthread.php?threadid=199
Personally, I don’t feel there’s much left to discuss here, but feel free.
not having seen the past discussions on this, my SWAG is…
You lower the pressure in your mouth, thereby causing the bit of the noodle right in front of your lips to be pushed in by the outside, higher atmospheric pressure. The rest of the noodle is light enough to be pulled along by the tension of the noodle itself (not directly by the pressure difference at your mouth). Like pulling on a rope, where the force is only directly applied to one end and the rest is just along for the ride.
Exactly Phobos.
I was heavily involved in at least one of those threads. I even conducted a pasta sucking experiment. At work no less.
Consider this: when you drink a beverage through a straw, what makes the beverage go up and into your mouth? Is it something grabbing the near end of the soda? No. Is it something pushing the far “end” of the soda? Yes - air pressure outside the top of the liquid pushing down, unbalanced by the pressure on the other side. Is there a ramp to slide the soda through the straw? No. Of course the soda does distort shape, but the soda does not have to make direct contact with the lips. You can stick the end of the straw into your mouth past the lips and it will still work. The soda slides along the inside wall of the straw.
Now for spaghetti. The noodle is getting pushed by air from the outside, in all directions. However, a small part (between your lips) has less pressure. Thus that part of the noodle moves, it gets pushed, and the rest of the noodle is pulled along by the tensile strength of the noodle. Thus there is another piece of noodle between the lips to get pushed, and another piece, until the end of the noodle is reached.
The sauce helps as a lubricant, but is not required (if you have moist lips). The softness of the noodle helps as a lubricant and helps it conform to the opening, but I think a crafty person can lube up a solid rigid noodle and suck it in, too. At least until it hits the roof of your mouth.
Air entrainment and deforming noodles need not apply.
I don’t this this theory can be correct. For the spaghetti to swell significantly, it would have to have an extremely low bulk modulus. I’d guess that the bulk modulus of cooked spaghetti and water are similar (about 20,000 atmospheres), so there’s no way you can get the spaghetti to change volume with the pressure difference you can generate with your mouth. It’s important to note that simply because a material can flex and change shape, it doesn’t necessarily mean that its volume changes easily - water is almost infinitely “flexible” but it doesn’t like to change its volume.
I read Cecil’s column but I am too lazy to read the other threads, so if this is repetitive, then damn me to hell. The claim by the original letter writer to Cecil that since the spaghetti buckles it can’t transmit the force due to air pressure is nonsensical. No matter how much the spaghetti buckles, there’s still a short length of noodle aiming straight into the mouth, even if the rest of the strand is hanging straight down. The air pressure against the back side of this length will tend to push it into the mouth. Think “projected area”.
I think there are other things going on, however. For one, if it’s just air pressure that does it, then why can’t you suck in a piece of uncooked spaghetti? This, I hazard, is because uncooked spaghetti is both dry and rigid, so it makes a poor seal against the soft lips. Also, notice that you make sort of a “splutting” noise as the spaghetti goes in. I think this is “stick-slip” - you build up a little vacuum in your mouth, developing some compression in the spaghetti, then the seal breaks, but the compression pushes the spaghetti a little way in. Then the seal re-forms, and the cycle repeats.
Off to Comments on Cecil’s Columns.
I suppose this needs to be said again–there is a difference between incompressible liquids and compressible spaghetti. “Ropes” are hard to push. Still, the amount of work that gets done (the effort of your mouth) is about the same for soda or spaghetti–true? I’ll still advocate for entrainment–that air/sauce is being drawn in on the sides of the spaghetti, and the friction on the spaghetti is drawing it in–but only because Cecil said he threw that out out of hand, and only until I see something definitive one way or the other.
RM, just how compressible is spaghetti? I know that ropes are hard to push - if you try to push from the end. However, suppose you wish to feed a rope through an eyelet, such as thread through a needle. While you can generally get the thread to act stiff enough to go through the needle, you don’t require the same thing for the rope. Just push the part of the rope directly at the eye, and it will go through. Then push the next part of the rope directly at the eye. Repeat. As long as you are pushing directly at the eye, you can push that part of the rope.
As for entrainment, again I will state that I actually went out and performed an experiment. Not a rigidly controlled and documented one, but I did several variations on sucking noodles. I can wholeheartedly say that entrained air does not work. If you try to suck a noodle but leave a bit of an air gap, it is very difficult to keep the noodle from responding to gravity and falling out of your mouth. If you have a seal between your lips and the noodle, the noodle slides in. Sauce makes for a good lubricant, but so does saliva.
If you doubt me, may I humbly suggest that you go suck on some noodles. (Anywhere else that would come off as an insult. 
)
Not entrained air, entrained sauce.
I’m having trouble imagining an experiment that would differentiate between the two effects. Ah, here’s one. I was going to have you poke a hole in your throat, thread an endless loop of spaghetti through that, back into your mouth…
Dang…no, that won’t work. I hope you haven’t started that one already. This is worse than the Bernoulli thread.
You guys are confusing incompressibility with rigidity. Rope is fairly incompressible, but it’s not rigid. That’s why you can’t push it. I think the same can be said of cooked spaghetti. Unless there is some evidence that air enters the spaghetti, I would expect it to have a very low compressibility, like water. But it’s clearly not rigid.
The reason you can push a decent length of thread but not of rope is that, because thread is so thin, you can’t help but push on it more or less straight-on. With rope, there’s a big enough diameter that you tend to apply a bending action to it. Also, thread is rigid relative to its weight and the forces needed to move it, while rope is not.
Actually I was not confusing the two, though with them run together like that I can see why it looks like it.
RM, here’s an experiment. Suck noodles without sauce. I know, I’m brilliant.
Interesting discussion, but try this experiment: suck a pen or pencil into your mouth. It still works like the spaghetti, but is rigid. So it must be a pressure differential, (lowered pressure in your mouth.)
You can get enough suction to move the pen into your mouth? Will you marry me?
Ignoring your immaturity (and thanking you for being the one to make that post :D). Sucking a pen into one’s mouth is child’s play, literally. I used to do it as a kid, and I just did it now – it wasn’t hard. I recall as a child doing this with rectangular and cylindrical blocks, again with little effort. [insert jokes about my homosexuality as a child here].
Sua
Yes, there is a pressure differential as there is for soda straws, even for rigid pens (and whatever else you put in that mouth), but how does that show that it is not entrainment for spaghetti?
RM, if we can suck liquids with pressure differential, no entrainment; if we can suck rigid solids with pressure differential, no entrainment (i.e. the pen, dry noodles, cooked noodles with no sauce); if when we suck noodles it is easy to suck if we make a good seal, but not easy to suck if we part our lips around the noodle so there is an air gap;
Then how can entrainment be involved? Why do you wish to add entrainment to cooked spaghetti when it isn’t necessary for anything else? Doesn’t the law of parsimony hint to discard it? Unless you have some evidence that entrainment is involved. Where is your evidence?
Until you justify that entrainment is necessary, I think I’m justified in not accepting it.
The law of parsimony is not a scientific justification. There’s no doubt that entrainment is involved–even air can do that–but we don’t know the size of the effect. I think Cecil’s reasons for dismissing this are over simplified. In fact, his “explanation” (that air close to the mouth has an inward-pushing component) is pretty close to a description of entrainment, wouldn’t you say? I’m actually leaning away from entrainment.
The point of the OP (Cecil’s) was that the spaghetti flexes. Sucking up a vertical pencil can be modeled by a body diagram where the forces balance on the sides, but not top and bottom. An equivalent would be to push the bottom of the pencil with your finger. With vertical limp spaghetti, that clearly doesn’t work. You can’t push up a strand of limp spaghetti. So, how does it work? Dismissing the question by saying that the answer is obviously air pressure differential is ignoring the question–even if that is right, as far as it goes.
RM, I’m not sure I understand your point. It’s pretty clear, at least to me, that the difference in air pressure causes an unbalanced force on the spaghetti. From basic physics (F = ma), the unbalanced force causes the spaghetti to move. Irishman’s point is that, since there is already an explanation at hand for why the spaghetti moves, there’s no need to drag another explanation into the picture.
I assume that you’re arguing one of the following: 1) The pressure forces on the sphagetti are balanced, 2) F<>ma, or 3) pressure forces alone are not sufficient to overcome frictional effects. True?
And, of course there is entrainment of the sauce. It’s just that it’s an effect of drawing the sphagetti into your mouth, not a cause.
None of the above.
Where does the force act? That’s the original question. That’s why the original letter writer wrote to ask the question. I haven’t seen a good answer posted to the SDMB, although I may have missed it. As I said, I’m leaning away from entrainment myself.
If you look closely at Cecil’s answer, he says it is probably along the spaghetti, near the mouth. That’s actually entrainment of the spaghetti by the fluid (air or sauce), even though he claims later to have rejected that theory.