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Jan. 24th, 2008 05:39 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
jackhttp://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.
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.
Re: How does air pressure turn into a sucking force?
Date: 2008-01-31 10:58 pm (UTC)Take a rigid cylinder, and put it between your lips, and suck (not for real, although you can if you like). The force on a side is the pressure times the area (where the pressure is scalar, and the force and area vectors). The forces cancel out on opposite curved sides, but there is more force on the higher pressure side than the lower pressure side, pushing the cylinder into your mouth.
If the question is "why is the force related to the pressure", the answer goes something like, the pressure is related to the energy/momentum/that kind of thing of the molecules in the gas. The force is caused by transfer of momentum/energy from the molecules in the gas bouncing off the solid. The higher the pressure (more molecules or higher molecule velocity), the higher the force on the solid. Sorry for not being more specific, I never got on with thermofluid things.
The key thing to remember with spaghetti is that it doesn't know it's spaghetti, i.e. where you perceive its ends to be is irrelevant. The change in pressure is over the section in your mouth/the projected ends of that cylinder on the outside of the spaghetti.
It does apply to a pencil as well.
Re: How does air pressure turn into a sucking force?
Date: 2008-02-06 11:44 pm (UTC)(1) Your description of pressure acting on the ends sounds like my description I just made of air molecules, in that if the force is acting up from the bottom, it doesn't correctly predict that spaghetti will be sucked up rather than buckle?
(2) Contrariwise, if you consider pressure acting across a small plug in your lips, surely that implies variable internal pressure within the spaghetti, which we both thought there wasn't?