Sucking Spaghetti

Cecil's Columns/Staff Reports
1

A while ago Cecil rejected the alt.physics crowd’s explanation about how sucking spaghetti works (a pressure differential) on the grounds that they didn’t say where the differential pressure was exerted.

As best I can tell, the answer is pretty simple. Given any line drawn through the spaghetti, the pressure on one side of the spaghetti (where the line goes in) is the same as the pressure on the other side of the spaghetti (where the line goes out) – UNLESS that line happens to begin inside the mouth, in a low pressure region, and end outside the mouth, in a high pressure region. There there’s going to be different pressure, and therefore unmatched force is going to be exerted along that imaginary line.

Therefore, pressure is going to be exterted at exactly those spots where you could draw an imaginary straight line through the strand and have it enter the strand outside the mouth and exit the strand inside the mouth.

The parts of the spaghetti that are flopping around inside or outside the mouth are irrelevant – pressure is equalized there.

Make any sense?

2

I refuse to get drawn into another endless debate on this topic. I will say, however, that the “imaginary line” argument is inadequate. Suppose you are sucking a strand of spaghetti while staring at the floor. The strand hangs vertically, and the imaginary line enters through the bottom end thereof. If you actually push on the bottom of said strand, however, it merely buckles and is not forced into your mouth. I stand by my original statement that “those [air]particles striking the spaghetti close to the point where it entered the mouth, and whose vector had some inward-pushing component, would force it in.”

3

Well, Cece, one thing you should remember is that refusal to get into an argument is incompatible with getting the last word.

The imaginary lines argument does work, despite your objection, for the following reason.

In the situation you described (vertical spaghetti), the instant you push on the bottom of the spaghetti, it bends, and the force you exert on the bottom of the spaghetti is diverted somewhere besides into your mouth. However, in the same situation, if you are sucking on the the spaghetti, if the spaghetti bends even a bit, the “imaginary lines” which determine where unequal force is applied fail to extend to the end of the spaghetti; the unequal force is exerted at some more relevant point, where the spaghetti bends, nearer the mouth.

If the spaghetti could not bend; if it were uncooked, for example, then yes, pressing the bottom of it would push it up into your mouth.

However, if it’s cooked and bendy, then the instant that it bends, the analogy between sucking (and producing unequal force) and pushing the end of it fails.

4

Hey Ed: That is what Cecil said sort of. If it is bendy, pushing on the bottom does nothing. I do not always read so good but no where in the previous post to your last post does he say that pushing on the bottom is what makes it go into your mouth.
As I see it says the blind man, “A partial vacuum exists in the mouth and the place of least resistance for something to enter to naturalize the difference in pressure is the small hole, for me that is, in the mouth and the spaghetti is in that hole so in it comes being pushed on all sides near the mouth by the air trying to get in also. Just like a can collapsing, the air is trying to get in so the weakest place is the one that moves. The spaghetti is the weak link.”

5

This has probably been said before, but regarding whether increasing the air pressure in the room would by itself force the spaghetti in, the answer is yes, if the person makes sure not to let the increased pressure buckle his cheeks.

If someone can come up with a clearer explanation of this whole imaginary line theory, please do.

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Cecil said:

I’m pretty sure that Ed has it right. The answer is that the spaghetti is never perfectly straight. If it were perfectly straight, then yes, it would start to buckle. But as soon as it starts to buckle, even just a tiny, imperceptable amount, the line of imbalanced force no longer goes through the end and it will not continue to buckle.

Forget about sucking spaghetti for a second and think about this situation. You have a cylindrical strand of cooked spaghetti lying perfectly straight and flat on the table. It had 15 lbs per square inch of pressure acting on all surfaces. So, on the flat ends, the pressure pushes on the left end towards the right, and on the right end towards the left. Does it buckle just sitting there on the table? If not, why not?

“For what a man had rather were true, he more readily believes” - Francis Bacon

7

I think there is a simple explanation that is eluding all you high foreheads on this spaghetti thing. When I am slurping a strand of pasta, I am usually not applying even, robotic, vacuum-cleaner-like suction, but am more likely to be moving my tongue and teeth to move the pasta along once it’s in my mouth. surely this exerts some drawing action on the pasta still outside my lips…

Live a Lush Life
Da Chef

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No, Chef, it’s all about unequal air pressure. That’s established. The only question was how that was applied to the spaghetti, and my answer was, the force is applied wherever (geometrically speaking) the inequality exists. If you can draw a straight line from one point on the spaghetti’s exterior surface to another point on its exterior surface, and air pressure is different at those two points (i.e. one is inside the mouth and one is outside it), then force is applied there.

I’m beginning to see why Cecil got so tired of this.

9

I’m always pretty amazed that people have trouble with this. The answer, of course, is that most people are making it too simple a model by applying too much math and not enough critical thinking.

We’re always tempted to look at things from a trignometry perspective, and look for a single net vector for the force on the pasta. But a single vector for anything is just an illusion. In reality you have millions of tiny vectors, one for each gas molecule that hits the pasta. The net vector for a strand of pasta being sucked into the mouth is upwards. However, very few of the actual vectors are upwards, that’s just the way they balance out.

Now, Cecil has the concept of the illusionary net vector mixed up here. He looks at a net vector pointed upwards and imagines that vector must be acting upon the base of the strand. It is not. The net vector is acting on the pasta at the portion of the strand which is being transfered from high to low pressure environments. The net vector is at your lips, not inches below them at the terminus of the pasta strand.

You can prove this to yourself with a very simple experiment, place a throw rug on a linoleum surface. The net force vector is zero, the rug is being acted upon by equal air pressure on all sides, by gravity above, and by the floor below. The vector from the floor is equal to the gravity’s vector. Everything balances out and the rug is stationary.

Now kick the very edge of the rug. You have introduced a new force vector. This force will act at the edge of the rug (ie. the base of the pasta) and the rug will fold over onto itself.

Now kick the middle of the rug. You have introduced a force vector in the middle of the rug (ie. at the point where the past passes the lips) which will cause the rug to move in that direction.

Lesson: The part of the structure to which the vector is being applied is just as important as the vector itself. They really ought to start teaching that as part of trig.

-Bob

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I was just thinking about that last post, and realized that I might not have been as clear as I could have been.

I meant that the imaginary line hypothesis (net vector) is not actually in conflict with Cecil’s assertion that it’s the gas molecules along the sides of the strand doing the work. Cecil applied the net vector to the wrong part of the pasta, and that’s where the misunderstanding seems to have come from.

-Bob

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Last thing I’m going to say about this. The problem isn’t MY misunderstanding of the “imaginary line” hypothesis, it’s others’ misinterpretation of what I said.

I never claimed otherwise. I merely observe that imaginary lines, net vectors, etc., are of little help in understanding precisely where the force on the spaghetti is exerted. For proof, see above thread.

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Net vectors no help? Perhaps you should rephrase that to say “imaginary lines, net vectors, etc., are of little help in understanding precisely where the force on the spaghetti is exerted to someone who doesn’t know where to place the net force vector.”

To those of us who do know where to place the net force vector, your conceptualization of tiny vectors at the lip section of the strand only recapitulates the information what we already had. It doesn’t provide anything new, it just states the obvious.

-Bob

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To those of us who do know where to place the net force vector, your conceptualization of tiny vectors at the lip section of the strand only recapitulates the information what we already had. It doesn’t provide anything new, it just states the obvious.

-Bob ]]]]]]]]]]]]

Hey Bob, Cecil never just restates the obvious. Show a little respect here.
::::::: mumble ::: obvious ::: mumbel ::::: grumble :::::::::

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Incidentally, one does not “suck” spaghetti. One “slucks” it. I have this on good authority, namely Garfield, who then goes on to complain about slucked-spaghetti whiplash.

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<—meekly raising hand. Does it matter if it is storebought, or freshly made? :slight_smile: ROFL
Typer

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I think that most everyone heard from on this misconceived the basic underpinnings of a good suck.

The mope who first raised this apparently threw people off with his notion that water sipped through a straw is drawn up the straw by a pressure differential. However, an efficacious flow of fluid is caused not by a
change in air pressure, but rather by the absence of such a change. To wit, a normally breathing human draws breath by contracting the diaphragm, thereby expanding the lungs and, because the air already in the lungs
occupies an essentially constant volume, additional air is forced in to fill the additional space. With lips sealed to straw, the column of air in the straw becomes, in effect, part of the respiratory system. When the diaphragm contracts, the lungs expand, and the air in the straw is drawn into the oral cavity, pulling the water behind it, just like a syringe.

The “syringe” principle works with liquids and solids (like spaghetti) in similar fashion (absent excess weight and excess friction). The diaphragm contracts, the lungs expand, and whatever substance is at the lips is drawn in to fill the additional space. Simple, no?

17

In reply to kjsheehan,
Actually the sucking action does cause a difference in pressure. The pressure at the surface level of the beverage must remain constant. Outside the straw 1 atmosphere of pressure is exerted on the beverage. Inside the straw, the air above the liquid, plus the column of liquid itself provide 1 atmosphere of pressure. So, the pressure above the liquid, in the straw is less than 1 atmosphere. This is why a straw will not work from a height higher than about 33 feet. To try to raise water up 33 feet, you would slowly have to reduce the pressure in the straw. At 33 feet, there would be a vacuum above the liquid, and it would be impossible to draw any higher. Of course this would require a very strong straw, and a really remarkable set of lungs. :slight_smile:

A column of mercury with a vacuum above it will stand about 30 inches, since mercury is much heavier than water. This is of course a barometer, a common way of measuring atmospheric pressure.
As for the spaghetti, everyone is saying pretty much the same thing, a diagram seems to be needed.
Dave

18

The primary problem seems to be a matter of simplifications. We all make them. The difference lies in the simplifications we each variously tend to make. Here is another view, that though perhaps initially counter-intuitive, may be instructive.

First, let’s examine the case of a perfectly rigid body. If we take the area encircled by the lips, then the ‘sucking force’ can be easily localized. it’s hard to explain this in simple terms but if you envision a cylinder extending out from the lips, and see where it intersects the external surface of the spagetti, then the force will be equal to the pressure differential times the perpendicular component of external surface thus described

This means that the force on the rest of the body is irrelevant. It all balances. For Cecil’s purpose, it only matters that a ‘region of force’ can be described. [non-mouthward forces can also be shown to cancel out, due to a fundamental geometric property of closed surfaces) This works for non-rigid bodies as well. But I believe that Cecil’s point is that ‘in real life’ no such region can be described. I respectfully disagree.

There is no such thing as a perfectly rigid object (and spaghetti wouldn’t qualify, even if there was), so instead of envisioning the stress (force), try imagining the strain (deformation due to the force). We’ve all seen ‘stress pictures’ made with polarized light (in full-color magazines or museums of science, even if you aren’t experimentally inclined)

Two examples of common blatantly semi-rigid solids are Jello™ and Nerf™. If we were to suck on a 1-foot sphere of either of the above, it is easy to envision what would happen: it would deform (bulge) in the region between our lips.

Is that where the force is ‘exerted’? At the mouth? After all, it’s where the strain shows up!

Naturally the answer is “Yes and No”.

The force is exerted on the OBJECT over its entire surface, creating an internal pressure. But the strain is caused by the difference between the INTERNAL bulk pressure of the strand (jello, Nerf, whatever) in the atmosphere, and the portion inside your mouth. This is an effect of the difference in air pressure.

Aha -so this whole fight is because we’re really talking about two different things: external pressure (force) and internal pressure (stress). If you measured you could see a pressure (strain) gradient over a 3 dimensional region in the vicinity of the lips. (Many visible household demonstrations are possible - try using a bowl of relatively transparent jello, and a 1-liter soda bottle with the bottom cut off to simulate spaghetti and lips. Strain can be visualized with two pairs of polarized glasses held at angles. Small bubbles in the jello can be seen to expand to differing degrees, as well)

Perhaps you find this explanation unsatisfying. Think of a bathyscape at 1500 meters. The water doesn’t seem to be doing anything. It’s just sitting there. the bathyscape doesn’t seem to be doing anything either, it is just resisting through its rigidity. However, let the tiniest leak develop, and it will become immediately obvious that tremendous forces are at work. The water jet could slice a limb off. But where is the force ‘exerted’ that pushes the water jet through the leak? On the surface of the water? No, it’s entirely internal pressure (roughly the weight of the water above the bathyscape = 1500 tonnes/m^2 = 1 ton/sq.in.)

If a bathyscape springs a leak on its bottom surface, does anyone have any difficulty seeing that a lethal force will be exerted upwards. If Cecil rejects this explanation because he cannot see ‘where the upward force is exerted’ would we need to re-examine how water pressure works?

I think not.

Pressure is a bulk property. It has several counter intuitive “emergent” properties as a result (Even something as simple as Bernouli’s principle can really surprise you sometimes) However, I don’t think that the spaghetti example contains anything very surprising.

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bump – cecil post