(no subject)
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.
no subject
Date: 2008-01-25 01:09 am (UTC)The air pressure in that axis acts on those projected areas (you may have to do maths with angles to prove this to yourself), to give equal forces in both directions. In the spaghetti case, imagine the end just at the mouth. the air is not acting on a small area, so there is an overall force.
Once there is some spaghetti inside the mouth, you can do the calculation for both parts separately. You'll end up with a force on from each side, but the higher pressure outside means more force, so it wins.
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But yeah, since it works with cooked spaghetti and not uncooked it must be the elasticity.
Solids will work by pressure differential alone for a large enough radius (e.g. pencil), because the cross sectional area (therefore force) increases faster than circumference (friction).
no subject
Date: 2008-01-25 06:02 pm (UTC)Imagine doing your flat-projection-force calculation for all of the spaghetti except the first inch approximately horizontally sticking out from your mouth. The air pressure on that part is the same whether you're sucking or not, but it moves when the first inch is sucked into your mouth, so where does that force come from? Must it not be along the strand of spaghetti?
no subject
Date: 2008-01-26 02:04 am (UTC)Say you had 1 inch sticking out North, a sharp bend, then a couple of inches going West. Ignore gravity (you are on SpaceShipTwo). It would be incorrect to state that the force is transmitted from the western end along the spaghetti, round the bend, then to the mouth.
This is hard to explain uninteractively. I need paper and a pencil to fight over, or maybe just hands to wave expressively.
no subject
Date: 2008-01-26 02:39 pm (UTC)*confused* Was I saying that? I didn't think so, but then I was thinking aloud.
no subject
Date: 2008-01-26 09:42 pm (UTC)Once the bit of spaghetti at the mouth is moving, the rest of it follows because it is being stretched. That force *is* along the axis.
The part I was trying to answer was "Why does it move at all?", or more specifically "Where does the push come from?".
no subject
Date: 2008-01-27 12:08 am (UTC)Why does it move at all?
Ah, right. Of course. I think, originally I was saying the force *wasn't* propagated all the way along from the end. If anything (assuming we were in a case where a vacuum *did* suck spaghetti) I'd guess the air acting on the first inch of spaghetti, maybe along the axis of that from the bend. However, I was fairly sure that:
(a) If you drew it all out with air pressure arrows and integrated it a good answer would fall out, and
(b) I was not capable of doing this articulately.
no subject
Date: 2008-01-26 02:39 pm (UTC)Oh yes, very. Diagrams?