Didn’t see a topic created on this yet (maybe I’m blind… it happens) So I thought I’d add my two cents. They didn’t seem to get a clear cut answer on how spaghetti is sucked into your mouth. To me it’s rather obvious, when you make the sucking motion… you create a vacuum in your mouth, and that drags the spaghetti in, which is hopefully well lubricated by sauce or else it’s going to drag on your lips hard. It’s so obvious that it’s a vacuum because what happens when you suck in? Your cheeks are brought closer together. Stick your finger in your mouth and suck… observer what happens.
The thing is: How do we know that spaghetti doesn’t collapse when you suck it in?
To be sure you’d have to try it in zero-G otherwise you don’t know whether it’s gravity that’s keeping the spaghetti straight.
I’m thinking maybe it does get bent a bit, but as soon as it does the air pressure is no longer in the same place and the bend straightens out. For example:
_____/-
A B
When the spaghetti was straight the pressure was on part B, but as soon as it bends a little the the pressure on B is equalized and it gets pushed from part A. This can happen very quickly and when you factor in that the spaghetti is accelerating towards your mouth (thereby straightening it the same way a rope gets straighter when you pull on the end) and it’s being pulled down by gravity you might not notice it bending.
Forget Physics. I’m an engineer. Try the Control Volume approach. Imagine a little 2D box, with the lips as the upper and lower boundaries, and two cross sections of spaghetti, one just inside the lips, one just outside. Limpness, rigidity…it don’t matter. Vacuum in mouth means lower pressure on inside. Lips are hopefully lubricated, so net force on box–> into mouth, with the friction of the lips preventing too much noodle speed.
Also, imagine if you will, an “unsuckable object” where this pressure diff. will not occur. YES! An empty straw, sucking on air! You’d get some air friction internally, tending to draw it into your mouth, but your lips will resist that.
Seattle Tom is very close… think about it this way:
When you create the vacuum in your mouth…what happens? Ever part of the wall is drawn in… including the spaghetti which is part of the wall… because your lips effectively form a seal around it…now it’s lubricated so it slides easily and is drawn in.
The first focuses on pressure and pushing (though I believe it is not as accurate)
Since the pressure at the sides of the spaghetti (outside of your mouth) always equals the pressure at the end (outside of your
mouth) it keeps it from buckling. Essentially the pressure at the sides acts like the walls of a pipe (or cylinder) and you can even
push a gas in a pipe.
The second explanation focuses on the fluid nature of the spaghetti:
Internal pressure in the spaghetti is equal to the pressure on the outside. If you then lower the external pressure on the spaghetti
(just inside your mouth) the internal pressure (in the spaghetti) just outside your mouth (caused by external pressure) pushes the
part, at your mouth, into your mouth. Cohesive forces (in the spaghetti) then keep the rest of the spaghetti in that space
between your lips until either the pressure inside your mouth equals that outside or until you are sucking air. 8-> (Essentially
then the only part that matters is the part AT your mouth.)
Seattle Tom… you’re in the right forest, but not the simplest tree. Think of a free body diagram of a cylinder. Draw a big arrow on the right end (that’s the outside atmosphere). Draw a little arrow on the left end (that’s the pressure inside your mouth. It still pushes against the spagHetti, but not as much). Now draw two little arrows above and below the cylinder, pointing to the right, representing friction. You can see that there is a net force to the left, into the mouth. Now, you say, that’sa obvious with a cylinder, because the two end planes are parallel, but ifa you were to take a solid, uncooked elbow macaroni, the FBD would be the same. In the dynamics textbooks they use a blob-shaped rock in the diagrams so you don’t get hung up with thinking you need perfectly parallel surfaces. Trust me that even with an amorphous, changing shape, the zillions of infinitesimal force vectors cancel out except for normal (perpindicular) to the cross-section in your lips. If they didn’t, a piece of cooked spagHetti would move by itself in a still room. (Remember Brownian motion?) And what nit invoked Heisenberg? When your only tool is a hammer, every problem becomes a nail.
SeattleTom: You’re right but that’s not the question. We know that it’s obviously the differnce in pressure that’s doing it but we want to exactly how. For example: why doesn’t the spaghetti buckle as it would if you were to simply push it in with your finger?
The question is exactly which air molecules are pushing it in? The ones near the mouth, the ones at the end… etc…
Absolutely! I didn’t include it in my post, but that’s what I was thinking. Even with the spaghetto hanging out of your mouth limply (and covered in sauce), a vacuum exerted inside your mouth will, adding up all the little force vectors even for a blobby CV, give you a net inward force on it.
I was taught to ALWAYS draw a free-body-diagram before attempting any such problem. You can simplify things so much it seems trivial later.
Imagine atmospheric pressure working on all the spaghetti strand outside your mouth. It exerts pressure uniformly in all directions, and the strand feels that pressure inside as well…we say it is also at “atmospheric pressure” inside and out. Equilibrium.
But at your lips, you can imagine a part of the noodle that feels less pressure. That cross section of material will drag the rest of the strand in, so you needn’t worry about buckling because you are pulling, not pushing.
Sorry to keep posting, but your last question about “exactly which air molecules” should be answered.
I can’t put my FBD here, but imagine a squiggly spaghetti strand sticking out of a (disembodied) mouth, with the strand cut off in cross section right between the lips. If you add up all the pressure vectors exerted by all the air molecules, all around the surface of the strand, they will all cancel, except for a few that have a component pushing into the mouth. The part pushing out of the mouth, exerted on the “cut” between the lips, is a smaller arrow, because of the lower pressure inside the mouth. So the net force exerted by atmospheric air (molecules) points IN.
Ok Ok enough already, the solution is so obvious it could have bit you all in the nose. Cecil, you should be ashamed at yourself for such an answer.
Pressure doesn’t just act normal (perpindicular) to the strand of spaghetti, it acts on all directions, setting up shearing (sideways) forces on the surface. Thoses forces are usally negated by other shearing forces right next to it, When the pressure drops, as the noodle enters the mouth, the internal shearing force is less in all directions but more importantly is less in the direction pushing the noodle out, The unbalenced force overcomes the frictional force (with the aid of a lubercating adn hopefully appitizing sauce)and the noodle slides in.
I don’t think that buckling is really a concern at all. Think about it this way: when you push part of the noodle, that part of the noodle has to go somewhere, right? And if there’s some more noodle in that new place, then that part of the noodle has to go somewhere, and so on. The only way this can stop is if this chain reaction of moving noodle parts goes in a direction in which there is no noodle. Then the noodle at the end will go into the reegion with no noodle, and we see the noodle as buckling. So take a vector representing a force on the noodle. Extend the the vector into a ray, and see if that ray exits the noodle at any place other than the end of the noodle. If so, then that force, unopposed, will cause the noodle to buckle. Noodles are so thin, and curve so much, that almost any time you try to puch the noodle, it will buckle.
Okay, now time for the explanation of sucking noodles. Take any molecule that hits the noodle. Extend its velocity vector to see where it exits the noodle. If that resulting ray exits the noodle on the outside of the mouth, we expect there to be another molecule with an opposing velocity vector that cancels the impulse of the first molecule. No net force, and so no buckling. Now, suppose the resulting ray exits the noodle inside the mouth. Assuming a near perfect vacuum inside the mouth, there will be no opposing molecule, and so there will be a force. But that force will act completely within the noodle. The noodle will not be able to buckle, and so will be forced into the mouth.
So, basically my explanation comes down to two points: 1. Under some circumstances, it is possible to push a noodle. 2. This is such a circumstance.
Thought this one was over. 2 engineering principals:
1Pressure is a scaler, not a vector, it has no direction.
2 you cant push on a rope
ryans explanation doesn’t hold up to this simple but fun experiment. get a noodle and a willing member of the opposite sex (my wife assisted me). you take one end and let her (or him depending on your case) take the other, let it hang limp between you, both suck and the noodle moves in BOTH directions. I found this a better experiment then my origional idea of an infintially long noodle. As I stated the pressure causes shearing forces along the length of the noodle, it is being pushed in from the sides, not the end.
I tend to agree with you that ryans explanation is too contrived. I think that it breaks down when he claims that opposing forces on a section of spaghetti will not cause it to buckle, at all. Maybe that’s true for a perfect incompressible homogenous cylinder, but probably not spaghetti. Just consider a wooden block that’s been cut in half by an oblique cut. Press the two ends together–it deforms.
On the other hand, I don’t agree with your unbalanced shearing forces theory, either. Those shearing forces are only unbalanced at a very small section of spaghetti, near the mouth–so small, that there really isn’t such a place. You’re going to have to go into more details, or else your theory stays in the category of “well, you suck, it goes in.”
possibly, but not yet, if I do indeed owe it, I will. But I think I got Cecil on the ropes on this one.
try this experiment at home ( all these carbs … well all in the name of science)
take a noodle and dip one end in butter, put the end with butter in your mouth so the butter section ends at your lips, let the un-lubed end just hang there. now take the end of the noodle and tap it(no sucking), it doesn’t go in further, now grab the noodle right by your mouth and shove it in - it moves in not much and you can’t apply much lip pressure but it does move (again no sucking). to make sure there is no sucking, I suggest taking a straw and putting it in you mouth in such a way as to allow pressure to equalize, preventing you from unintentionally equalizing
