(no subject)

[jack]

http://www.straightdope.com/classics/a4_184.html

Straight dope asks "How can you suck a strand of spaghetti?" The question being, sucking a liquid, you create a vacuum in your mouth, and it's generally enough to say the pressure within the liquid is less than that in the mouth, so a force acts on the liquid in the straw. Sucking a solid object (eg. pencil), you can say the air pressure cancels out all over, except down the length.

But spaghetti is floppy. The air pressure on the *end* of the strand can't be relevant, because pushing their wouldn't force it into the mouth.

The answer doesn't seem very satisfactory. I'm sure it's something like, air pressure generally acts all over the surface, perpendicular to it, and this cancels out all over[1]. Except on a line through the part of the strand through the lips. So there's a force on that part, propagated down the strand to the next bend (where it acts sideways to the strand).

But I can't really put that into words (or symbols). Can anyone else provide a simple, satisfying description?

[1] May be hard to show, either by common sense or integrals, but we know it *does* because the net air pressure on a strand of spaghetti in midair (neglecting variations with height) is zero everywhere.

Comments

[hairyears.livejournal.com]

Yes, you can suck spaghetti... Cooked spaghetti. The part of the strand inside your mouth expands, as it is no longer subjected to atmospheric pressure. However, this expansion is not a straight step: the expanding portion trapped between your lips forms a curving slope, with the sloped portion pressing outward against your lips, both radially and axially, with the axial force propelling the spagetti inwards into your mouth.

You can do this with soft and floppy compressible cooked spaghetti. The rigid uncooked strands do not expand significantly in your mouth, no matter how hard you suck, and the atmospheric pressure on the very end of the strand exerts a negligible force compared to the friction at your lips.

[cartesiandaemon.livejournal.com]

Hm. Two erudite and opposite opinions in as many minutes. I've just tried it with raw spaghetti and contrary to all my expectations it doesn't seem to work Can anyone else confirm this is right?

I expect you're right, and that sounds like it could be, but do you know that for sure, or it just seem the obvious reason to you?

[hairyears.livejournal.com]

No, I just thought about it for a minute, made an effort to recall what it feels like, and stated the obvious.

Googling for the physics of sucking spaghetti reveals no convincing references for my hypothesis.

[cartesiandaemon.livejournal.com]

A brief experiment shows spaghetti only comes in when: it's cooked, I compress my lips, and suck. It's not properly double blind, but two seem insufficient. Of course, I never do suck spaghetti, anyone who has would probably/apparently/maybe know all that already.

That seems to support your theory (though not 100% conclusive).

[cartesiandaemon.livejournal.com]

PS. I emailed the Straight Dope, linking to this thread. It will probably be ten years before they respond, but it would be good to do out bit fighting ignorance.

[cartesiandaemon.livejournal.com]

Hm, and cooked spaghetti *does* work.

[hairyears.livejournal.com]

A simple refutation of the 'manipulation' theory is to coat the spaghetti in oil (or pesto, which is tastier): note how much easier it is to suck in, and how much harder it is to 'mumble' it up into your mouth.

Now repeat the experiment with cold spagetti that's been left out in the air for half an hour: it's still floppy and compressible, but the surface is sticky. Try sucking that in! Manipulating it inwards with your lips is now easier, but still far less efficient than applying suction.

This does nothing to prove or disprove my theory of how the suction works.