Fishy Spaghetti Explanation

Cecil's Columns/Staff Reports
1

In the How does one suck in a piece of spaghetti? classic column, the explanation of what is happening on the molecular level does not hold up,

Yes, most air particles hit obliquely, but as we all know, for every molecule that hits at a given angle, another hits from the opposite direction at the same angle, so any force parallel to the surface sums up to zero leaving only a net normal force per area, the pressure. In the billard ball world of molecules, this is pretty much true everywhere right up to your mouth. Even if you consider edge effects when you get down to the mean free path, that’s less than 100 nm in air at room temperature and pressure. That would be a LOT of force per unit area on that tiny strip of spaghetti to be able to push it.

However, another set of thought experiment puts makes it clear that air on the surface cannot be the mechanism. We all know about sucking liquid up a straw. People seem to think the rigidity of a straw has something to do with it. It does not. Consider sucking water up a flexible hose. We all know that works fine too.

So next consider sucking a piece of spaghetti through a tiny, flexible hose. Now I have not actually done the experiment, but I am confident that it would work fine. Once again, trying to physically push on the end of the spaghetti would not push it through the hose. The hose would just give. In this case, there is no air pushing on the lateral surface of the spaghetti anywhere. In fact, friction between the hose and the spaghetti will make it harder to suck it through, but you still can do it.

So… Now explain what is happening on the microscopic level sucking the spaghetti through a hose? Now ask yourself, is the physics really any different whether the hose is there or not?

2

Though I maintain that the answer will eventually boil down to a mathematical summation, I do believe the question has great pragmatic interest on a molecular scale (including a few military application I can postulate) but I don’t feel that these can be readily explored on a layman level, nor would they satisfy the teeming Millions, who frankly are not going to be mollified by an answer that only applies to spaghetti.

“But what of gummi worms?” they will cry, and then run down a shopping list of objects that only share broad principles of material properties "What of Lumbricus terrestris (the earthworm) or wet shoestrings or… well, let’s just say that Cecil may not suck, but my experience with the Unwashed Masses in the Emergency Room is that they do suck – with infinite variety, but little discretion. Eventually some wannabe sci-geeks with altogether inadequate empirical experience will declare that sexual intercourse is impractical, and probably apocryphal, due to their analysis of theoretical pump action and suction effects.

Or somesuch. (The problem with Darwin is that he is too darn slow)

What the Teeming Milions really want --nay, demand-- is a generalized mechanism.

So such I shall provide.

First, let us establish that a rigid bar can be sucked into a mouth. Diamond, plutonium, candy cane – it matters not-- but Jello, likewise, can be sucked, as can a stream of water from a water fountain or bathwater from a tub. This suggests that rigidity is a red herring. The force of the air on the end of a candy stick is not what pushes it into the mouth.

I can prove this.

Let us posit a near-zero friction liquid (some friction is needed to preserve conventional fluid properties. If you want to mess around with sucking experimental assemblies containing, say, Helium superfluids, well, it isn’t medically recommended (see Jearl Walker’s essay on his experiences with cryogenic fluids in the human oral cavity in the inestimable Halliday, Resnick textbook “Fundamentals of Physics” – which he now co-edits) or just take my word for it: bad idea

Take a hollow ball. Drill a hole in it that fits your rigid rod (say a candy stick) reasonably well, but not perfectly – it should not establish a perfect seal. Fill the ball with your low-friction fluid and insert the rod part way, displacing some fluid, and leaving no air. Create a second identical assembly for use as a control. We will assume sufficient surface tension to barely retain the the liquid, and keep it from pouring out of the stick/ball assembly when they assembly is placed in a horizontal orientation. C----

Place the free end of your test rod in you mouth. Hold the control rod in an identical orientation in a test stand. Suck on the test rod. It pulls into your mouth, but the external air pressure differential is not being transmitted through the distal end, because that end is covered by your ball, and any net force on the ball would squeeze more fluid out of the ball, before it was transmitted to the rod, yet you can suck the ball-tipped rod into your mouth as readily as an untipped rod.

A physics or mathematical analysis will show that --sucked or not-- the forces on the fluid in the ball are balanced, except for inertial pseudo-forces due to the low velocity of the assembly being sucked into the mouth (in any suitable frame of reference). To control for the pseudo inertial effects, move the control assembly in exactly synchrony to the sucked assembly – heck, link the two assemblies with frame of diamond or infinitely rigid unobtainium to guarantee identical motion. Sucked or not, any leakage will be identical. Sucking one end of a rigid rod doesn’t appreciably alter the balance of forces on the other.

If there is not net change in force on the ball, and the low-friction lubricating oil (oe water or whatever) is not squeezed out of the ball when you suck the other end, the force on the candy cane must be more proximal to the mouth. It must be exerted on the sides of the rod.

If a rigid rod isn’t really being pushed by unbalanced forces on the distal end, the lay conception is utter crap, and we can dismiss it for either a rigid rod or flexible spaghetti. Who cares if the spaghetti flops when you poke the end. You can suck it just as well by putting a water-filled ball on the distal end. The distal end is being pulled not pushed.

Let’s “sciencify” it even more, and use a suction pump to suck a free-floating globule of water into a rubber hose. If the globule were being pushed distally, it would visibly compress in the direction of motion, due to the unbalanced force on one side. If the net force were spread over its entire surface, then it would shrink symmetrically as its volume disappeared into the tube. This experiment was done, albeit as an amusing demo for school children, rather than as a well-funded research program by the pasta conglomerates – guess what happened? At moderate velocity, the globule remained roughly spherical as it disappeared into the tube. It didn’t flatten from being pushed in the direction of motion.

3

I’m sorry, but I didn’t read half of the hair brained explanations.
It just seems to me that this is a very simple problem, and everyone seems to be bringing very complex concepts to explain it.
Let me try explaining it in simple terms.

When you suck in general, with nothing else in your mouth, air goes into your lungs. Why? Because your lungs expand, creating a vacuum that needs to be filled. K?
Now, clog your mouth in some way, and try sucking in some air. What happens? Well, nothing happens, because air cannot enter, your lungs cant expand. But if you put something there that doesn’t clog, be it, air, a pencil, hell, even some spaghetti, it gets sucked in to fill that vacuum. When you are attempting to create a vacuum, one of 2 things must happen. It gets filled, or it doesn’t. Since spaghetti can’t clog your mouth, it instead fills the vacuum, and gets sucked. Is that not a good enough answer?

4

No, because it doesn’t answer the essential question, which is “Where and how is the force being exerted that pushes the spaghetti into the mouth?” This is a different question, and an important one.
Although it’s a different situation altogether, Consider the case of a helium or hot air balloon. It rises. Howcum? The easy explanation is that its density is lower than the surrounding air’s, and answering it that way will satisfy some people, and is analogous to your answer for the spaghetti. But if you look deeper into it, you ask "Well, if it’s accelerating upwards, there must be some force pushing on it. Whrere is that force? If the surrounding atmosphere is exerting force on all the sides of the ballooon equally (and to first approximation, you can model your balloon as a perfect sphere), then the balloon shouldn’t move – all the forces will equal out. If you consider it carefully, it’s evident that the forces can’t be equal on all sides. And they aren’t. The force of gravity (which we know has to be responsible for the balloon going up – we invoked it in the “simple” explanatiom) has to be responsible. Gravity causes a pressure differential between the upper portions and the lower portions of the balloon. You can actually calculate the effect mathematically, using that spherical model, and get the required answer - the force that drives the balloon upward is the gradually decreading pressure on the sides of the balloon as you go upwards. And so your understanding of the universe is improved.
So, in this case as well, there must be an explanation for the force responsible. By analogy with the balloon case, it simply HAS to be that the sum of forces outside your mouth acting on the spaghetti exceeds those inside, forcing the spaghetti in. However:

1.) This case hasn’t got anything to do with gravity. I haven’t done the experiment, but I’ll bet dollars to spaghetti that astronauts in the space shuttle ccan such in spaghetti while in orbit.

2.) It doesn’t have anything to do with quasi-fluidic properties of spaghetti. I’ve sucked it unyielding objects just as easily.

3.) But the spagheti case adds an interesting wrinkle – it’s hard to believe that pushing on the limp far end of the spaghetti adds much to the driving of the spaghetti into the mouth.

4.) External air pressure HAS to have something to do with it. Otherwise spaghetti could as easily spontaneously slip into your mouth when you’re not sucking at it. One expects that a shuttle astronaut in orbit WITHOUT the shuttle couldn’t suck in spaghetti, even if he had the presence of mind to attempt the experiment under those circumstances.
So exactly where the forces come from isn’t completely clear, except that it has to be a vector summation over the outside of the spaghetti. If you imagine the spaghetti cut off just outside your mouth, the transverse forces ought to be the same, and it’s clearly in part the forces on the end of the nub of the spaghetti that forces it in. Add a little more to the spaghetti and the situation is the same. All I can suggest is that as you lengthen that nub of spaghetti, the longitudinal forces on the sides start to add up, replacing the mostly longitudinal push from the end of the strand, so that when the strand becomes long, the pressure o n the end doesn’t add much and isn’t needed to push it into your mouth.

5

Cecil’s answer isn’t “fishy.” It’s just plain wrong. Cecil’s answer was that “Those 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.” However, this explanation is, well, silly. Any place on the surface where air particles strike has air particles striking in all directions. The force from any particle having an inward-pointing velocity is (statistically) canceled by a nearby particle having an outward-pointing velocity. So: forget this answer for the time being.

I think the confusion here arises because people tend to misapply their intuition (which is usually reliable) to this problem. Any thought of “forces” and “pushing” leads to the inevitable “you can’t push on a rope” argument. People forget that air pressure is not equivalent to a single force; it is a *pressure * exerted everywhere. So: forget your intuition for the time being.

So where does the air pressure force act? Or, more properly, how does the force act? Thought experiment: an astronaut floats a piece of spaghetti in front of him in the Space Shuttle (no gravity = easier thought experiment). Does the spaghetti move? No; the air pressure forces are balanced (No Force, no massXacceleration). Now imagine a tiny cubic chunk of the surface of the spaghetti. There is air pressure on one side of the cube; the other sides are buried in the spaghetti. Does the cube move? No; the air pressure force is balanced by compressive forces within the spaghetti material.

Under air pressure (a hydrostatic load), these compressive forces (stresses, really) are equal in all directions within the spaghetti. You can slice the strand in any place at any angle, and the internal stresses are all equal to the air pressure. You can prove this by another thought experiment: The aforementioned astronaut cuts the spaghetti in some random place at some random angle. Do the two halves suddenly fly apart? No; the forces still balance; the air pressure now between the two halves is equal to the internal stress that was there before the cut. No change.

Now, for the sucking of the spaghetti. Another thought experiment: The astronaut gently places the strand in his mouth and lightly closes it. Air pressure everywhere is constant; internal stresses are constant. Cut the spaghetti just outside the astronaut’s mouth. There’s no change except air pressure has now replaced internal stresses in one plane. But, here’s the key: There is no place in the spaghetti strand that is affected by the cut. All the forces are still the same.

It should now be intuitively obvious that when the astronaut starts sucking, the very very short strand of spaghetti will be pushed into his mouth by the difference in air pressure. And this case having a short strand is equivalent to the case of a long strand.

Or, if you prefer, it’s the difference in compressive stresses outside and inside the lips that causes the strand to suck in: the stresses are caused by the air pressure.

Does that make sense? The explanation is perhaps a little more convoluted than Cecil’s, but it is (ahem) correct.

6

Your thought experiment has a different result than mine under the same circumstances (cited in the previous post) – I think that the spaghetti in the absence of gravity WILL move.
We’re gonna have to get someone in zero-G to try this out

7

Different thought experiment, I think. My though experiment is just a piece of spaghetti–no lips, no sucking–which has uniform air pressure all around it.

8

Sorry – missed that. But in that case, your explanation seems to be exactly the same as mine.

9

OK, this one really has my noodle fried, and I generally consider myself to be a Pretty Smart Guy when it comes to physics. My wrinkle, if you will:

Spaghetti has a certain amount of sproinginess to it, to use the technical term. If I have a sufficiently long piece of cooked spaghetti, one end in my mouth, and the other end grasped firmly in my finger tips a couple of feet away, I can still suck in a fair amount of spaghetti without moving my hand. The noodle simple stretches, and some of the spaghetti goes into my mouth. Clearly this has nothing to do with the force on the tip of the spaghetti in my fingers, because that’s not moving at all. However, it’s definitely air pressure… the air is applying a force on the noodle-cylinder in all directions, and when I create an area of low pressure, the pressure outside my mouth squishes the diameter of the noodle in just a tad, and that force acts to push the noodle into my mouth. Sort of like squeezing a tube of toothpaste.

Now, if you take the firmly gripped end of the noodle out of the equation and let it dangle freely, I can still see some of this effect going on. In this way, I’m able to grasp how air pressure can push a non-rigid object into my mouth.

Does that do anything for anyone, or is my noodle still fried?

10

Okay, I made a few really shity video aids :slight_smile:

Case1: ridgid spaghetti
Imgur

Case2: half of spaghetti cooked
Imgur

Case1: Surface striking particles, cancel out. B cancels C and D cancels E in all pressures.

A:Since surface area of ends are the same, if pressure is equalized, same number of molecules will be of type A and F,therefore force will be equalized, and no movement.

b:If chamber(mouth) lowers pressure(sucks), there will be more molecules of type A, than type F and the noodle will move into to the mouth.

Case2: Again Surface striking particles, cancel out. B cancels C and D cancels E in all pressures.

The outside end is no longer a simple circle, It is a curving three dimensional surface, of greater area. This gets into the oblique angles, more molecules are hitting it now, but each one gives less force in the lateral direction . Although harder to see, all other molecules cancle out each other, leaving the same basic inward force as was there was with ridgid spaghetti. If pressures are equal they will still counteract each other, and there will be no movement. And if the pressure in the mouth is lowered, it will still suck in.

Case3:Fully cooked noodle[Okay I suck couldn’t draw a picture worth a damn.]

But you can view it as a specialzed case of Case2. The the molecules traveling through the original A area are this there, And they are not canceled out by aything on the outside. You can’t just declare them equalized somewhere.

11

Kids, this just isn’t that tough. Air pressure acting on all points of a piece of spaghetti sums to a net force of 0 when the strand lies on your plate. By reducing that pressure at one point, you reduce the air-pressure contribution in that direction, making for a net reactive force in the direction of decreased air pressure. The most obvious practical proof of this is that you can also push spaghetti strands out of your mouth by just increasing air pressure in your mouth.

I would guess that since the spaghetti hangs down from your mouth, the pressure exerted at the bend in the strand just on you lower lip contributes the most direct “push” into your mouth when you start to suck, but this is immaterial. You must consider the total pressure over the entire strand before determining if the spaghetti strand will move.

Those intrepid scientists Lady and the Tramp proved in the 1950’s that, when two people/canines suck on different ends, (1) each end of the spaghetti will move into each mouth and (2) the strand’s center of gravity–the point at which the researchers inevitably share a kiss–remains (more or less) fixed. Point (1) demostrates how the resultant summed force from the air pressure acts locally at each end of the strand, while (2) shows that the summation of pressure over the entire strand is, as expected, 0.

12

Come on, folks! It’s not that complicated, and you don’t have to reduce it to a molecular level to explain it.

Purse your lips around the tip of your pinky finger; the contact zone is about a quarter inch (about 6 mm, for the metric among us.) Place a short piece of spaghetti into your pursed lips, and increase the pressure in your mouth by gently blowing. The spaghetti moves from greater pressure to lesser (atmospheric) pressure, and it pops out.

The limpness of the spaghetti is a red herring. If you take a full size piece of cooked spaghetti, and grasp it gently with a pair of tweezers a quarter inch from one end, you can push that short end all around the plate. The long end follows, of course.

There’s roughly 15 pounds per square inch air pressure on you, the plate of spaghetti, and everything else in the room. When you put one end of the spaghetti into your pursed lips, it still is equal pressure everywhere. Then, you draw the floor of your mouth downward, and the pressure in your mouth drops. Your pursed lips are the only low pressure outlet in the whole room. All the air in the room wants to move to a place of lower pressure, but the portal is blocked by a piece of spaghetti! That first quarter inch rushes into your mouth, but the portal is still blocked. The air in the room continues to shove the spaghetti into your mouth until the whole strand is in there. All that air still wants to go to a lower pressure, but you (you big tease) equalize the pressure. There’s no more imbalance, so the motion through your pursed lips stops.

CJJ* said pretty much the same thing, but I already had my spiel wound up, so I posted it. One or both of us will make it all clear. Maybe.

13

Heck, I said it back in post #4.

14

Yes, you did, once you got past the helium balloon stuff. Even so, some folks will read yours, CJJ*'s, and mine, and they still won’t get it. :smack:

15

Any point on the surface has air striking it from all directions that at least partially point towards the interior of the spaghetti. So the net force due to air pressure on that piece of surface is perpendicular to that segment of surface, and directed inward.

Obviously these are canceled by the internal pressure of the spaghetti so that it doesn’t collapse in on itself. But in terms of determining the overall acceleration of the strand, all that matters is the external forces. Cecil’s point is that there’s a net horizontal acceleration, so there must be a net external force in the horizontal direction. There are horizontal forces on the hanging strand, but the forces on opposite sides of the strand cancel out. This leaves the horizontal force pointing directly into your mouth from the bend in the spaghetti (see wolfman’s second picture.)

Understood in this way, I think that what Cecil is saying is basically correct.

16

Let simply that to bullet points:

  • The overall acceleration of the spaghetti depends on external forces applied to the spaghetti.

  • The spaghetti has a net horizontal acceleration, so there must be a net horizontal external force

  • This horizontal force is due to air pressure

  • If we treat the inside of your mouth as a vacuum, the only segment of the spaghetti strand that experiences a net horizontal force due to air pressure is the segment right in front of your mouth.

So that’s what Cecil means when he says “Those particles striking the spaghetti close to the point where it entered the mouth … would force it in.”

17

Ehm, Cecil said what all you guys said. What’s the deal?

18

You guys are missing the really important part of all this:

Note how Cecil says:

But if you check the Google Groups archive of sci.physics, the question is posed by one edzotti @ aol.com.

Cecil taking the credit for his editor’s work? Surely not… :wink:

Edit… OK, so I was beaten to it.

19

Except at the boundary where the lips meet the noodle’s surface. At that boundary, only inward-striking air particles can strike the noodle. The only outward-pointing particles that could strike the noodle at that point are lip particles, which are constrained by cell walls and whatnot.

There is an opposing boundary inside the mouth where outward-striking particles strive to balance the outer, inward-striking ones. But you’re sucking, so there are fewer air particles (or none, if your mouth is full) per nanometer of boundary. The inner, outward particles are outnumbered and overwhelmed. The noodle is sucked.

Now, for the matter of Rotini…

20

If the noodle was attached to a treadmill…