another factor that I belive to be a minor one, but still there it that the noodle expands if less pressure (then atmospheric) is applied. this expanding will draw the noodle in.
is the noodle under tention or compression in the lip zone, this will tell us what force is dominiant. any one got an ediable strain gage???
by squeezing a cylinder , the sides only, not the 2 circles at the end, the clyinder lengthens. Now from my 1st experiment (which possibly led to the conception of our 1st child, we will see in a few weeks) we see that pressure on the end ‘doesn’t do jack’ (CA). so the propelent force must be near the part that is nearest the lips (or a pulling, but we are not going to accept the ‘you suck, it goes in, theroy’. now the trick is the substance we are working with, it has characteristics of a liquid and a solid (semi-solid)., how does it act at the lip interface. I am going to assume that the noodle acts more like a solid with a very low k value (very elastic, like rubber. where F(force)=k(elastisiticy constant) * x(distance of deformation). the squeasing near the interface is balenced on one side by the squeesing right next to it but imbalenced on the other side bu a pressure drop, the material lenghtens into to mouth.
cant continue right now, out of clean dishes. maby one of you can pick up where I left off.
looking back at Cecils origional response, it seems that we have come to the same conclusion about the sidways force of air pressure. so you are indeed corect an apology is owed.
Cecil, I defentally owe you this apology. and realze the error of my ways as I assumed what you said and didn’t go back even though the link was provided to me, please forgive my evil ways. you must realize that a student must chalenge and one day surpass the master (or the master is just a teacher and we all know what a ‘great’ job the public schools do). that being said, the issue is not resolved.
k2dave
Master would be proud–good post, that last one.
I’d like to see more consideration given to the very last theory presented at that link. Cecil says it was the first thing he thought of, and the first thing he rejected. I think he might have been a little hasty. The theory was:
The reason Cecil thinks that this is improbable? “Given the minute amount of fluid and the minimal cross section it presents, it’s implausible to suggest that the sauce or whatever is the sole or even the primary medium of propulsion.” That isn’t sufficient reason to reject the theory, in my opinion.
Everybody seems to understand how a fluid like water or spit or sauce can be sucked into a mouth–liquids transmit pressure, solids usually don’t. The pressure differential forces the liquid in–no matter how minute the amount of liquid or how minimal the cross-section. Once we’ve established that, the next step is a little trickier.
When fluid dynamicists study fluid (liquid or gas, according to them) flow past solids, they find a small layer very close to the solid surface called the Ekman layer. That’s where shear friction forces are important. In fact, close to the solid surface, such friction is considered to be very great, effectively infinite. Your mouth would have to pucker small enough to involve the sauce Ekman layer, which draws the spaghetti with it.
That would explain why, if you open your mouth wider, the noodle stops. No matter how much you suck, no matter how great the pressure differential between the outside and the inside, the noodle doesn’t move in. It’s only when you constrict the opening around the noodle does it move.
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Don’t ask. I don’t even know what I meant by this
not so sure of the drawn in with the fluid theroy. I would kick around some assumptions to see if we are all on the same playing field
1 the force that causes the motion is basiclly equal in magintude the net diffrence between the forces on either end due to pressure minus effests like friction, gravity and possibly a normal force caused by the expanding noodle as it enters lower pressure. (I put possible because I know that point is contriversial and I who believe in this force don’t think it is significant)
2 the force that actually casues the motion is caused close to the lips and not at the end.
The problem is that Ryan is assuming an ideal noodle. (Does that sound as silly as I think it does? He, he, ideal noodle…)
Right now, I think the 2 most probable explanations are
- Is is buckling but we don’t notice it
- One side is less compressed and by expanding it’s being sucked in
The best way to test this is to do an experiment with something compressible and something non-compressible and measure the difference in pull. I guess a hot dog will emulate a noodle, and that’s on a big enough scale that it’s easy to measure.
Just as I was coming to grips with the likes of Control Volume, ‘k’ value and Brownian motion, there sits Konrad’s last post, urging us to:
“do an experiment with something compressible and something non-compressible and measure the difference in pull. I guess a hot dog will emulate a noodle, and that’s on a big enough scale that it’s easy to measure.”
The picture of this research actually being carried out is stunning in the extreme.
Pardon the injection of psychoanalysis here; but I can only fervently hope – to paraphrase Freud’s famous dictum (sorry) – that*“sometimes a hot dog is only a hot dog.”*
aw $#!+, I had a hotdog for lunch, and this experiment slipped my mind, probally setting back humanity by a good day or two.
but there is a problem w/ the hotdog test. you can push on a hotdog. M<aby it;s not your favority after work activity but it can be done.
If you think of the environment the spaghetti has to move in, I think it gets a little easier. Why shouldn’t the motivating force on the spaghetti be found as simply as the pressure delta times the cross section of the spaghetti strand at the mouth?
Envision it this way: If one of the Teeming Millions were to put the very end of a spaghetti strand in his mouth, atmospheric pressure would be available equally to all sides of the strand except the very end, which would only be subjected to the partial vacuum inside the Teeming Millionth’s head.
The model is the same if the strand is halfway in/out of the mouth, or if we are only discussing air. The spaghetti needn’t be involved at all if we are only looking for the force available to act on it. We have two pressures and an orifice, why make it difficult?
I think it would even be possible to derive an equation to create a model of the acceleration rate for the strand of spaghetti, provided you are willing to make (too many) assumptions about the pressure differential, the coefficient of friction between the mouth and strand, effect/existence of air leakage, volume of the mouth, and the volume of the spaghetti strand.
[The model is the same if the strand is halfway in/out of the mouth, or if we are only discussing air. The spaghetti needn’t be involved at all if we are only looking for the force available to act on it. We have two pressures and an orifice, why make it difficult?]
we have discussed that the propellent force is the magnitude of the difrence of the pressure forces plus and minus other factors. the question is where it the force that actually does the pushing. it cant push on the end since that does nothing. also take my earlier experiment with 2 people sucking each end of a limp piece, there is no x-sectional area exposed to atm pressure.
I am comming to the conclusion that the noodle is pressurized to 1 atm by pressure on the sides. I am assuming that if we take the noodle as a liquid (semi solid), then the pressure is uniform within the noodle. the lower pressure inside the mouth causes lower noodle pressure. Inside the noodle there is on one side small pressure vs large pressure —><<<=======, there is your force imbalence. this is a new theory, would like to kick it around the sdmb awhile…
I am, of course using Conrads ‘ideal noodle’ in my above post (forgive me, I just had to use the phrase ‘ideal noodle’)
Atmospheric pressure works equally on all sides of the spaghetti. When the sucker starts to create a pressure differential, the only side of the strand that the atmosphere can’t work on is the cross x of the piece in the mouth. Then the strand moves in the direction of the gradient.
Thought experiment:
Imagine a lovely model puts a marshmallow in a completely empty two-liter bottle with no lid, and it falls to the bottom. Then she knocks the bottle over and stomps on it. The marshmallow will not move because pressure acts equally on all sides.
Then, after blowing the bottle back to its normal shape (in as ladylike a manner as possible), she inserts another marshmallow in the mouth of the bottle. This time, when she pounces on the bottle, the marshmallow will be shot out. 'Cause pressure acts on all sides but one. Same as the spaghetti.
Cecil said: “The one place where the air pressure is not counteracted is on the end of the spaghetti.” This is not correct. The pressure on the end of the spaghetti is counteracted, but not by the pressure on the other end. It is counteracted by the pressure acting on the curved portion of the noodle directly across from the end. In other words, if you drew a straight line into the end of the noodle, it would intersect the side of the noodle at the first place that it curved. The pressure acting over this curved portion creates a force that is perpendicular to the surface. If you took the force vector at each point along this curved portion, and broke it up into components parallel and perpendicular to the end of the noodle, you would find that the sum of the components perpendicular to the end exactly balances the force due to the pressure acting on the end.
The same thing happens at the end of the spaghetti at your mouth, except that the force on the end inside your mouth is smaller because of the lower pressure. So the force due to the pressure on the curved portion just outside your mouth is not exactly balanced, resulting in a net force pushing the noodle into your mouth.
This is easiest to visualize if you draw the spaghetti as a couple of straight segments, instead of a curve. With some geometry and trigonometry, you can find the force acting on each straight segment (both on the inside and outside of the curve), and you can add up all of the components of those forces that are perpendicular to the end. You will find that they exactly balance the force due to the pressure on the end.
This is basically the same idea that Toch23 already mentioned, but I thought I could clarify it.
I agree…everybody is getting to the solution but from a different angle.
BTW, pressure DOES only work as a vector, normal to the surface, because all the side forces cancel each other out! So I don’t buy the “shear” argument.
The Maint. Guy’s thought experiment is just the same as my arguments. Basically, the vacuum in your mouth PULLS on a cross section of spaghetti and that’s that.
Don’t overcomplicate things! This is an easy one!!!
Seattle Tom
I’m just not sure I believe the explanation. If it were only the pressure, would the spaghetti move into the mouth if the mouth weren’t wrapped around it? In other words, what if there was a pressure drop from one side to the other, but the opening was larger. Would the spaghetti still move as quickly into the mouth? I’m not so sure. Can you demonstrate that?
From RM:
Methinks the reason is the opening would allow air to flow, and thus rush past the noodle experiencing friction with the lips. If there is no opening to allow air past, then the only way to reduce pressure is to move the noodle into the mouth so the opening is exposed and thus air can get in.
Here is a similar experiment. Take a boiled egg, an erlynmeyer (sp) flask, a piece of paper, and a match. How do you get the egg into the flask? Fold the paper, light it, and drop it in the flask, then set the egg on the opening. The paper burns, using up the oxygen in the flask, dropping pressure. The egg squeezes through the opening into the flask, and thus allows the pressure to equalize. How do you get the egg back out? Blow into the flask, raising the internal pressure, and the egg squeezes back out. It’s all by pressure difference. A boiled egg is similar in consistency to a limp noodle.
-Any resemblance to an actual physics experiment is intentional. I done seen it done.
irishman
I’m not denying that the difference in air pressure has something to do with it. I’m just not sure that it is everything. I’ve done the egg experiment, and yes it works. That is egactly the sort of thing that we think of when we say “you suck, the noodle moves.”
But you can maintain a pressure difference (with a large enough pump). Will the noodle move, in an opened mouth? I’m not so sure.