# Fishy Spaghetti Explanation

When I was a lad, I had a practical joke toy that consisted of a squeezy ketchup bottle and a piece of red cord - the cord was threaded through the nozzle and knotted at either end to prevent it going right through.

The joke was that you pushed the cord all the way in (it was several feet long), so that only the knot was visible on top of the nozzle. Then you squeezed the bottle and the red cord would be very rapidly extruded out, and would appear to your victim as if it were a stream of ketchup being squirted at them.

That’s how spaghetti gets into your mouth - it’s extruded (intruded, I guess) through your lips because the whole of the outside environment, including the spaghetti itself, is at a higher pressure than the inside of your mouth, and there’s somewhere for it to go - so it does.

Lip particles?

Which makes me wonder: If one end of the spaghetti has negative pressure applied to it and the external portion of the spaghetti has positive pressure applied to it, is the external portion of the spaghetti sucking the atmosphere (as a result of a transfer of energy through the spaghetti from the inside of the mouth) or is the atmosphere blowing the spaghetti? If the atmosphere is blowing the spaghetti, then from whence does the kinetic energy applied come from if not from the spaghetti sucking the atmosphere?

Either the spaghetti sucks (like my ex-wife’s)
Or the rest of the world blows (which should go without saying)

OK, I’ll play along here. How wide to you think this boundary where “only inward-striking air particles can strike the noodle” is? How large an area is that on a nominally-sized piece of spaghetti?

There’s no such thing as negative pressure. There is no suck, only blow.

I dunno - probably a couple of square millimetres if we think of it that way, but at potentially several pounds per square inch, that’s still enough force to move a piece of slippery spaghetti.

For the actual width you’d need to ask an actual physicist instead of a compulsive writer. He or she (the physicist) might say the force is highest at the actual point where lip touches noodle, and decreases rapidly farther away, perhaps inverse-logarithmically.

Which I suppose is a lengthy way of saying I don’t know.

OK then, let’s try this another way.

Suppose an air particle bounces off the spaghetti within this magical boundary zone. Where do you think this air particles goes after it bounces off? What path does it follow?

Now suppose another air particle travels along that path in exactly the opposite direction. Will it hit the spaghetti? Where will it hit?

I’m not even sure what you’re driving at? Is this just incredulity that something as seemingly insignificant as bouncing air particles can exert palpable forces?

The same seemingly insignificant process keeps vehicle tyres inflated.

It’s incredulity that there’s a region “where the lips meet the noodle’s surface” where “only inward-striking air particles can strike the noodle.”

mwbrooks is arguing that there’s some region there the air pressure doesn’t act perpendicular to the surface of the spaghetti, but rather there’s a component of the pressure that’s tangential to the surface, and that’s what drives the spaghetti into your mouth. I thought you were agreeing with him, to the extent of describing this non-perpendicular-pressure area was “a couple of square millimeters.”

But now I think you’re not agreeing with him, but interpreting his statements incorrectly. Or maybe I’m interpreting his statements incorrectly. One or the other.

zut, the fact that the spaghetti moves into the mouth (i.e., in the horizontal direction) indicates that there is a net force on the strand pushing in that direction. So on at least some segment of the spaghetti, the forces pushing into the mouth overwhelm the forces pushing out of the mouth. I can only see how you’d have that net horizontal force on the segment right in front of the mouth.

Here’s a doodle to illustrate. The red curve is the mouth, the blue curve is the spaghetti strand, and the black lines are possible trajectories of air molecules hitting the spaghetti. Note that I only include air molecules originating from outside the mouth. (Some particles would strike the spaghetti from inside the mouth, but the lower air pressure inside the mouth means that there are fewer such particles, and the ones hitting from outside the mouth dominate.)

You object that the particles travelling along the opposite trajectory produce an opposite force. This is incorrect. Suppose the particle hits directly horizontally. It flies in to the right, and flies out to the left. The time-reversed trajectory is identical – flying in to the right and out to the left. (If it helps, think about video-taping the particle and then watching it in rewind.) So there is only a net force to the right.

Now, suppose the particle hits the spaghetti at an upward moving diagonal. (See the picture.) The time reversed trajectory is the downard diagonal. But in both cases, the horizontal component of the particle’s velocity is to the right before the collision, and to the left after the collision. Thus the net force is to the right.

Unfortunately, your doodle requires a log-in for the rest of us to see it. Can you post it to somewhere public like Flickr or Photobucket?

Ignoring the randomness caused by molecular interaction, the particle will go back the way it came, more or less, after bouncing off the noodle and then the lip. Just like a tennis ball thrown towards a corner comes back at the same angle.

The difference is that particles that hit the noodle first hit it with a little more energy than those that hit the lip first.

Here’s a thought experiment for you: lay a throw rug in a doorway and close the door, then point a tennis-ball-throwing-machine at the bottom of the door. Count the number of throws that hit the rug first versus the door. If you set the machine so the two hit counts are the same (and if the floor is slick enough), the rug will gradually slide out of the room.

I admit that was an oversimplification. I could say only inward-striking air particles can strike the noodle without striking a lip first. That’s still somewhat simplified.

I am assuming that air molecules lose some energy when they bounce off less energetic things. I guess I’m also assuming lips are less energetic than air.

Right now I’m suppressing various observations about lips, energy, and sucking in general. I’ll leave those as an exercise for the reader.

Yes, I know a net force is required for the spaghetti to accelerate. I disagree with mwbrooks, however, that the unbalanced force is produced because “only inward-striking air particles can strike the noodle” “at the boundary where the lips meet the noodle’s surface.”

He is arguing, in other words, that “the air pressure doesn’t act perpendicular to the surface of the spaghetti, but rather there’s a component of the pressure that’s tangential to the surface” (that’s my paraphrase, not his, to be clear). Since you earlier said, “the net force due to air pressure on that piece of surface is perpendicular to that segment of surface,” I don’t think we’re in disagreement. Unless you’re saying something else entirely, which is possible.

No, I didn’t. I was actually, in a (apparently lousy) Socratic way, implying that there is no zone “at the boundary where the lips meet the noodle’s surface [where] only inward-striking air particles can strike the noodle;” in fact, there are just as many outward-bound particles striking the noodle there.

Ah. OK, then, you realize that you’re arguing that air particles slow down every time they hit something, right? So what happens over time, as air molecules get slower and slower? To simplify, what do you think happens in a closed container? Do the molecules eventually slow to a stop?

Certainly they slow down, if there isn’t any external energy source. You can demonstrate that yourself by heating up a tin can, sealing it, and letting it cool down. The can will collapse as the air molecules in it lose energy.

You can’t really cool molecules enough to stop them, or at least if you did I gather you couldn’t tell exactly where they were. Frankly, it’s out of my depth, but I think a Bose-Einstein condensate is as close as anyone has gotten.

Well, yes, actually, they will, if the environment of the bottle is already at absolute zero.

So temperature is important, yes? How does that factor into your conception of molecular interaction at the lip-spaghetti interface? Is it impossible to suck in cold spaghetti?

…which a strand of spaghetti isn’t. How is this relevant?

The speed of the air molecules corresponds to their temperature. I believe you can say an air molecule that bounced off a less energetic molecule is cooler than it was before, but I’m not well versed in thermodynamics.

It might be impossible to suck spaghetti if the air is extremely cold. I don’t know. The spaghetti or your lips might freeze before the got cold enough to make a difference.

Here’s a related question: Can you suck spaghetti in a vacuum? I’d guess not.