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Featured B Vacuum or pressure to move spaghetti through a hole

  1. Dec 17, 2016 #1

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    Hello,

    I have a somewhat dumb question. It is based on the fact that I can create a small vacuum in my mouth to move that "string" of spagetti into my mouth.
    I know it is possible, but I do not understand how spaghetti is possible to suck in if the shape is even and round like a "piston".
    I have learned that pressure is acting angular to any surface. With the spaghetti, that surface will cause the pressure to act 90° to it, and (in my thoughts) not be able to create a force that pulls or push the spaghetti into my mouth. But still it does. Why?

    Vidar
     
    Last edited by a moderator: Dec 31, 2016
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  3. Dec 17, 2016 #2

    Drakkith

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    Good question. My first thought is that spaghetti is far from being "airtight". There is gas bouncing around inside it as well as outside of it, and I wouldn't be surprised if that helped. Unfortunately I don't know if that's actually a good answer or not.
     
  4. Dec 17, 2016 #3
    I don't think we need to consider the pressure distribution along the whole surface of a spaghetti string. I believe the lips play important role. The combined effect of the lips+ of vacuum created in mouth makes the spaghetti being pulled like someone is pulling it from one edge.

    Regardless if my reply is satisfying or not, I believe you should put the spaghetti inside your mouth with the fork, doing minimal sucking, its not polite to suck spaghetti all the way from one end to the other :)
     
  5. Dec 17, 2016 #4

    CWatters

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    The partial vacuum inside your mouth doesn't "pull" the spaghetti into you mouth. It's the air pressure outside that pushes it in. If there was a vacuum outside you wouldn't be able to suck the spaghetti into your mouth.

    The spaghetti doesn't have to be air tight. It's soft so the pressure inside the spaghetti is the same as the air pressure that surrounds it. You could say the air pushes the spaghetti and the spaghetti pushes itself into your mouth.
     
  6. Dec 17, 2016 #5
    I've heard this theory many many times, and I still can get it; how is it that the outside air pressure is that strong? if I make a vacuum the pressure is really strong, how is it that it doesn't feel that strong normaly without any vacuum? then, the air in my mouth is more pressed than the outside air. Do you know about any experiment that prove this?
     
  7. Dec 17, 2016 #6
    Hmmm ... this post surprised me. Air pressure is an extremely well known and common part of every day life. I know of several "experiments" the most obvious being that air pressure is what a barometer measures. It is what those front lines on the weather map are showing. You test it every time you blow up a balloon or suck on a straw. It is caused by the weight of the atmosphere. We are at the bottom of a vast ocean of air, and exactly the same as how the pressure increases when you dive to the bottom of a pool here at the surface of the earth we experience 14lb /sq. in. of atmospheric pressure.

    The reason you don't feel it is because you are built for it. 1 atmosphere is our normal. If we were thrown into a vacuum we would be very uncomfortable. (to say the least)
     
  8. Dec 17, 2016 #7
    Returning to the OP. The question was about shape.

    Consider a circular opening in a wall with a pressure difference across the wall. If the opening is plugged with a small disk, we easily see the force pushing the disk through the opening. The question is if we plug the opening with a long cylinder how does it get pushed through the hole. The forces on the sides of the cylinder do not push the correct direction.

    I think the OP's instinct is correct. If the hole is plugged with an infinite straight cylinder there is no force pushing the cylinder through the hole. Therefore the reason we can slurp spaghetti is either because it is not infinite, or because it is not straight.

    It is easy to see how any finite straight cylinder still gets pushed through due to the pressure on the ends. However that doesn't answer the question for a long strand of spaghetti.

    I think the trick is to consider a bent piece of spaghetti. Suppose we have a cylinder plugging the opening, but the cylinder is bent to have a straight section through the opening and then sharp 90 degree bends on either side. Now once again we can easily see the unbalanced force. Of course real spaghetti won't make exactly that shape, but any amount of bend has a similar result.

    So unless your spaghetti stays straight and is infinitely long it still gets sucked through the hole.
     
  9. Dec 17, 2016 #8

    CWatters

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    Most people cook their spaghetti so it's soft when they eat it. Therefore the pressure inside the spaghetti is the same as outside it. So there is a pressure drop at the point where it enters the mouth..

    spaghetti.jpg
     
  10. Dec 17, 2016 #9

    CWatters

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    That's not really the answer. It's because the pressure inside us is the same as the pressure outside.

    When the pressure outside reduces (eg we go up in an aircraft) the pressure inside us changes to match. The human body is mostly water so it doesn't have to expand much to rebalance the pressure. However some parts of us, like the sinuses, are filled with air, and that has to expand much more to rebalance the pressure, for that reason we can sometimes feel pain in the ears or bloated until some air escapes.
     
  11. Dec 17, 2016 #10

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    Thanks for your replies. I didn't expect this much response to an apparently silly question :-)
    Well, I still don't get it. I can understand that if I replaced the spagetti with a thin iron rod (Shorter one but same diameter). The outside air pressure will push the rod in my mouth by the force excerted on the rods flat end, but the spagettis end does not push the spagetti in. So what force does?

    The mouth will make the spagetti thinner between the lips, and even if the spagetti is swollen up by the vacuum inside the mouth, I think it won't explain the force that sucks the spagetti in. I'm lost at this point - still after your replies. Maybe the swallen spagetti push back on the firm lips, but that wouldn't explain how to move the spagetti piece in both directions. At this point, in spite of being 44 years old, I feel very immature, hoho :-)), but this physics really bugs me.
     
  12. Dec 17, 2016 #11
    This has nothing to do with axial pressure forces on the spaghetti. There is a small annular gap between your lips and the strand of spaghetti. Air is being sucked into your mouth through the annular gap. You can hear this air flow when you are sucking in the spaghetti. The air that is being sucked in through the annulus creates a viscous shear stress on the spaghetti, tangent to its surface. This provides the force to drive the strand into your mouth.
     
  13. Dec 17, 2016 #12
    That sounds reasonable. However, I don't do that and the spaghetti still goes in.
     
  14. Dec 17, 2016 #13
    And yet if you drop the pressure further all sorts of bad things happen. Primarily water vaporizes (and other dissolved dissolved gasses come out of solution) forming bubbles and gas pockets. They say it doesn't officially boil, but basically enough vaporizes until you blow up like a balloon and the elastic tension equalizes the pressure. You swell up painfully to something like twice your size. You can't hold breath in your lungs because the pressure difference actually would do serious damage, so you'd better exhale before being tossed into space. The heart can't effectively pump blood due to the gas and the distended veins. You pass out in ten to fifteen seconds. You die in something like 90 seconds. So while we can live happily in a range of pressures from Death Valley to Mt Everest (well, most the way up Mt. Everest anyway) we are not built to function at or near zero pressure. We are constructed to live on Earth in an atmosphere of pressure give or take.
     
  15. Dec 17, 2016 #14
    I see. I believe you are saying that the spaghetti has a low enough bulk modulous so that it expands as it enters the mouth. If it does I can see how that would do it.
     
  16. Dec 18, 2016 #15

    CWatters

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    It doesn't need to expand. Water sucked up through a straw doesn't expand (at least not significantly). Your lips act just like a very short drinking straw. There is a pressure difference between the ends.
     
  17. Dec 18, 2016 #16

    Nidum

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    +1

    The same principle has applications in industry . Most common use is for feeding fine cables and filaments .
     
  18. Dec 18, 2016 #17
    Warning to the bro's:

    I tried reproducing this experiment at a Japanese restaurant using small bore rice noodles in a watery soup base while on a first date. Varied the shape of my mouth cavity to optimize the effect.

    I went home alone - thanks PF

    :-(
     
  19. Dec 18, 2016 #18

    OmCheeto

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    I tried to produce an experiment, both last night and this morning, as your assertion struck me as incorrect.

    Last nights experiment ended poorly, as I wasn't able to insert my noodle into my "non-mouth" orifice, and, like the time I was going to disprove "bananas are yellow", I ended up eating my experiment.

    This morning's attempt ended up as usual, with me:
    a: hurting myself: spaghetti is hot!
    b: getting very angry: this syringe does NOT mimic my mouth!
    c: thinking: I need two jello type apparatuses
    d: digging through the cupboards: only to find my jello-molds are the wrong size.......​

    But, I did find some test-tubes. And I know where to buy jello.

    Videos of experiment attempt #3 at 11!
     
  20. Dec 18, 2016 #19
    Has anyone factored in sauce viscosity. It will effect the sheer tensor.

    Assuming we are not doing this sauce less.
     
  21. Dec 20, 2016 #20

    OmCheeto

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    With Jello!
    again.....
    Jello makes for a very poor noodle.

    again
    Though this time, I decided eating A LOT of Jello for breakfast was a very poor idea.

    Yesterday's attempt ended up as usual, with me:
    a: shooting jello out of some fish tank tubing, which bounced off the ceiling, and leaving remnants scattered around my kitchen*
    b: getting very angry
    c: thinking, I need to think about this some more
    d: discovering that it's nearly impossible to get gelatinized jello out of a test tube, without destroying your jello noodle
    2016.12.20.more.Jello.experiments.png

    *This might make an interesting high speed video for @Andy Resnick . It was quite explosive, once the pressure exceeded the adhesive force of 'jello' to 'fish tank tubing'.
     
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