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#1
04-24-2012, 10:00 PM
 The Flying Dutchman Guest Join Date: Nov 2000 Location: 50N West Georgia Strait Posts: 8,600
Sucking, and I don't mean through a straw

We all know the scientific mechanism that allows us to suck liquids through a straw, but what about sucking in a limp spaghetti noodle ? I can't believe that atmospheric pressure can push that limp noodle into my pursed lips any more that I can push my limp...............
#2
04-24-2012, 10:08 PM
 Carptracker Guest Join Date: Jan 2009 Location: Missouri Posts: 152
Quote:
 Originally Posted by The Flying Dutchman I can't believe that atmospheric pressure can push that limp noodle into my pursed lips ..
Believe it. It does. Why can you not believe this?
#3
04-24-2012, 10:16 PM
 beowulff Member Join Date: May 2001 Location: Scottsdale, more-or-less Posts: 15,161
It's not pushing from the end, like your limp...
The spaghetti is being pushed from all over, especially right near your lips.
#4
04-24-2012, 10:25 PM
 Dewey Finn Charter Member Join Date: Apr 2003 Posts: 25,273
Some of your questions might be answered in this Straight Dope column.
#5
04-24-2012, 10:40 PM
 cornflakes Charter Member Join Date: Aug 1999 Location: Bubbaville Posts: 4,757
Cecil kinda covered this one. Personally, I think that the spaghetti is pulled in by the difference in pressure between the atmosphere and inside your mouth. If I'm doing the math right, sucking on a 1/16 inch thick strand of spaghetti should pull the strand into the mouth with about 1/10 ounce of force (I'll leave it to someone else to test this. )
#6
04-24-2012, 10:44 PM
 Trinopus Guest Join Date: Dec 2002 Location: San Diego, CA Posts: 22,852
Quote:
 Originally Posted by Carptracker Believe it. It does. Why can you not believe this?
It's a little counter-intuitive, especially when we think of forces involving linear vectors passing through the center of mass of the object being accelerated. The idea of a whole lot of little forces being "toe-nailed" (illustration for disambiguation re toe- and finger-nails) takes a kind of leap of visualization. We're more accustomed to the "billiard ball" model of forces and acceleration.
#7
04-24-2012, 11:00 PM
 Gary Robson Charter Member Charter Member Join Date: Mar 2003 Location: Montana, U.S.A. Posts: 9,449
Moderating: Moved thread GQ->CoCC/SR

[moderating]
Since Cecil covered this, I've moved the thread to "Comments on Cecil's Columns" for further discussion.
[/moderating]
__________________
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Yes, I have joined the ranks of former moderators. Being a mod was eating my life. Now I'm a member just like you. Except smarter and better looking.
#8
04-25-2012, 06:22 AM
 cornflakes Charter Member Join Date: Aug 1999 Location: Bubbaville Posts: 4,757
Quote:
 Originally Posted by Trinopus It's a little counter-intuitive, especially when we think of forces involving linear vectors passing through the center of mass of the object being accelerated. The idea of a whole lot of little forces being "toe-nailed" (illustration for disambiguation re toe- and finger-nails) takes a kind of leap of visualization. We're more accustomed to the "billiard ball" model of forces and acceleration.
Why wouldn't air molecules be pushing the other way after interactions with the molecules of our lips?
#9
04-25-2012, 07:54 AM
 zut Charter Member Join Date: Apr 2000 Location: Detroit, MI Posts: 3,725
Quote:
 Originally Posted by cornflakes Cecil kinda covered this one.
Yes he did, and gave a particularly shitty answer.

Previous threads on this subject may be useful:
Fishy Spaghetti Explanation (1/2008)
Spaghetti Physics (12/2000)
[url=http://boards.straightdope.com/sdmb/showthread.php?t=582]The Spaghetti Incident? (12/1999)
#10
04-25-2012, 08:58 AM
 penumbrage Guest Join Date: Jan 2008 Posts: 57
Quote:
 Originally Posted by The Flying Dutchman We all know the scientific mechanism that allows us to suck liquids through a straw, but what about sucking in a limp spaghetti noodle ? I can't believe that atmospheric pressure can push that limp noodle into my pursed lips any more that I can push my limp...............
Can you believe you can suck spaghetti through a (close fitting but not tight) straw?
Can you believe it still works even if the straw is just 1/4 inch long and form fitting to the pasta?
I'm afraid I can't help you with your other problem, you'll either have talk your significant other into sucking or shop around for a new one who will.
#11
04-25-2012, 09:13 AM
 The Flying Dutchman Guest Join Date: Nov 2000 Location: 50N West Georgia Strait Posts: 8,600
Here's what I think.

Close your mouth and with a noodle of spaghetti starting from the front of your mouth, you can pull it through with your tongue and the roof of your mouth to the rear. Care must be taken to not over tension the noodle or it will pull apart.
No atmoshere is involved.

Really, I just figured it out.

Of course we can not apply tension to liquids, but we can with solids.
#12
04-25-2012, 12:40 PM
 Powers Charter Member Join Date: Jun 1999 Location: Rochester, NY, USA Posts: 1,017
Look, we can solve this by simple experiment. Place a human being and a plate of spaghetti in a vacuum chamber and see if he or she can still suck a strand up.

Powers &8^]
#13
04-25-2012, 01:51 PM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
Place someone in some kind of pressure chamber, have them seal their lips around a piece of spaghetti, then increase the pressure. I don't think the spaghetti will slither into their mouth.
#14
04-25-2012, 01:59 PM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
On a more serious note, do the above with a test tube with a rubber stopper. Drill a spaghetti size hole in the stopper, and place a wet piece of cooked spaghetti through the hole so that most of it is still hanging outside of the test tube.

suspend the whole thing spaghetti down in a pressure chamber, then increase the pressure and see if the spaghetti is sucked into the test tube.

Anyone have access to equipment this could be done with?
#15
04-25-2012, 02:03 PM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
Maybe this is something that could be suggested to Mythbusters.
#16
04-25-2012, 04:51 PM
 Irishman Guest Join Date: Dec 1999 Location: Houston, TX, USA Posts: 12,252

Take something thick but soft, like the palm of your hand, and hold it up to your lips. Begin sucking. You will notice an inclination for the skin to pull into the mouth through the lips. This does not on the face of it seem to be by external air pressure pushing the skin into the mouth. The hand is fairly thick, and the tissue soft enough but bones rigid enough that it does not seem to be the same as if a thin, flexible membrane were held to the opening.

So what causes the movement? It is tempting to say that the suction creates a pulling force. But how does that work?

Suction is created by inhaling, which expands the lungs, and creates a larger volume for the air that is in the mouth/lung system. If the opening of the mouth and nose is sealed, the result is the same amount of air in a much larger space, or a lower air pressure. Why does this lower air pressure pull on the material over the opening?

If the mouth opening is sealed with a thin membrane, it is easy to say that the suction is really the pressure difference inside the mouth vs outside air, and the movement comes because the external air pushes the membrane into the opening.

But take a thick, rigid surface that is solidly mounted, like a wall or an item of furniture. Now put your mouth up to the surface (feel free to clean the surface to your content prior to this exercise). You create a suction force that holds the mouth to the surface without the surface moving. The rigid nature of the surface means it is not being deformed, and the solid mounting means that air pressure on the far side is irrelevant. What causes the force that secures your mouth to the surface, the "suction"?
#17
04-25-2012, 05:11 PM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
Maybe it's the air pressure on the back of your head?

As for sucking on your hand, don't your bodily fluids have pressure? Those pressures are presumably in stasis with the air pressure, so when the air pressure on a portion of your hand is reduced, the fluid pressures push it out.

Last edited by davidm; 04-25-2012 at 05:12 PM.
#18
04-25-2012, 08:12 PM
 Trinopus Guest Join Date: Dec 2002 Location: San Diego, CA Posts: 22,852
Quote:
 Originally Posted by davidm Place someone in some kind of pressure chamber, have them seal their lips around a piece of spaghetti, then increase the pressure. I don't think the spaghetti will slither into their mouth.
Um... Why not? It seems to follow logically. It would certainly work that way with a cup of soda and a straw.

And, actually, both this and the vacuum chamber experiment are feasible...
#19
04-26-2012, 08:48 AM
 Powers Charter Member Join Date: Jun 1999 Location: Rochester, NY, USA Posts: 1,017
Quote:
 Originally Posted by Trinopus Um... Why not? It seems to follow logically.
Yes, but not intuitively.

Powers &8^]
#20
04-26-2012, 08:53 AM
 Happy Lendervedder Guest Join Date: Dec 2001 Location: Michigan Posts: 13,581
Quote:
 Originally Posted by The Flying Dutchman We all know the scientific mechanism that allows us to suck liquids through a straw, but what about sucking in a limp spaghetti noodle ? I can't believe that atmospheric pressure can push that limp noodle into my pursed lips any more that I can push my limp...............
...noodle into your wife's pursed lips.

Sorry, that adds nothing to the conversation but I had to finish the thought.
#21
04-26-2012, 10:22 AM
 Irishman Guest Join Date: Dec 1999 Location: Houston, TX, USA Posts: 12,252
Quote:
 Originally Posted by davidm Maybe it's the air pressure on the back of your head?
This is where an experiment needs to be devised that makes two hard-mounted fixtures, one of which creates suction and presses up against the other one. And we need some kind of force gauge to measure the suction force.

Quote:
 As for sucking on your hand, don't your bodily fluids have pressure? Those pressures are presumably in stasis with the air pressure, so when the air pressure on a portion of your hand is reduced, the fluid pressures push it out.
That is an excellent point. The interior forces of the material in question are also part of the issue. That is why I shifted the example to a hard object like a cabinet or wall, something that does not have fluid inside that can deform easily.

This is definitely a thought provoking topic.
#22
04-26-2012, 10:23 AM
 zut Charter Member Join Date: Apr 2000 Location: Detroit, MI Posts: 3,725
Quote:
 Originally Posted by davidm Place someone in some kind of pressure chamber, have them seal their lips around a piece of spaghetti, then increase the pressure. I don't think the spaghetti will slither into their mouth.
Quote:
 Originally Posted by Trinopus Um... Why not? It seems to follow logically. It would certainly work that way with a cup of soda and a straw.
Quote:
 Originally Posted by Powers Yes, but not intuitively.
Why not intuitively? I mean, is it a) disbelieving that a pressure differential will move a limp noodle, or b) thinking that increasing the pressure on one side is somehow different thatn decreasing it on the other, or something else entirely?

Quote:
 Originally Posted by Irishman But take a thick, rigid surface that is solidly mounted, like a wall or an item of furniture. Now put your mouth up to the surface (feel free to clean the surface to your content prior to this exercise). You create a suction force that holds the mouth to the surface without the surface moving. The rigid nature of the surface means it is not being deformed, and the solid mounting means that air pressure on the far side is irrelevant. What causes the force that secures your mouth to the surface, the "suction"?
But air pressure in general is not irrelevant. Try your thought experiment in a vacuum; there will be no "suction."
#23
04-26-2012, 10:32 AM
 Irishman Guest Join Date: Dec 1999 Location: Houston, TX, USA Posts: 12,252
You certainly need air pressure in your mouth and lungs to suck. I'm just trying to figure out how it works when dealing with rigid objects.
#24
04-26-2012, 10:54 AM
 Darth Panda Guest Join Date: Mar 2010 Location: NC Posts: 5,681
Quote:
 Originally Posted by Irishman You certainly need air pressure in your mouth and lungs to suck. I'm just trying to figure out how it works when dealing with rigid objects.
Hiyoooooooo!
#25
04-26-2012, 11:05 AM
 zut Charter Member Join Date: Apr 2000 Location: Detroit, MI Posts: 3,725
Quote:
 Originally Posted by Irishman You certainly need air pressure in your mouth and lungs to suck. I'm just trying to figure out how it works when dealing with rigid objects.
The same way it does with non-solid objects. Creating a vacuum in one area around your head creates an imbalance in the pressure forces on your head, and that imbalance needs to be balanced in another way. It can be balanced with contact forces between your lips and the wall, and, if you pull back slightly, by forces in your neck muscles.

The forces on the wall will be balanced in the same way, through the contact forces on your lips or through the connection to the ground.

I think the thing that might be confusing you is that the wall has a connection to the ground that the saran wrap does not have, so if the forces are transmitted this way, they're a bit more obscure. It might be easier to think of a suction cup on the wall, where the vacuum "suction" is more obviously balanced by the contact forces.
#26
04-26-2012, 11:58 AM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
Think about a suction cup on a wall. You place it against the wall and push to remove the air. It then sticks to the wall because of atmospheric pressure. It sticks because there is no air (or a lower density of air) between it and the wall.

Now take a round flat disk of the same material as the suction cup. Press it hard against the wall. Will it stick? No. Why? There's little or no air between it and the wall. There's atmospheric pressure against it. So what's the difference?
#27
04-26-2012, 12:26 PM
 zut Charter Member Join Date: Apr 2000 Location: Detroit, MI Posts: 3,725
Quote:
 Originally Posted by davidm Now take a round flat disk of the same material as the suction cup. Press it hard against the wall. Will it stick? No. Why? There's little or no air between it and the wall. There's atmospheric pressure against it. So what's the difference?
The difference is that there's not "little or no air" between your disk and the wall. Even a finely polished surface will have surface features on the order of 0.01 to 0.1 microns (10-8 to 10-7 meters). Molecules of air are measured in the single digits of Angstroms, or 100 times smaller. Your typical painted wall probably has surface features in the 10 - 100 micron range, or something like 100,000 times the size of the air molecule.
#28
04-26-2012, 12:36 PM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
Okay. So why isn't that true of a suction cup against the wall. (I'm not trying to be difficult, just trying to reason out exactly what's going on.)
#29
04-26-2012, 01:57 PM
 zut Charter Member Join Date: Apr 2000 Location: Detroit, MI Posts: 3,725
A suction cup has a little hollow between the cup and the surface. When you push on the cup, the hollow flattens and pushes air out. When you relax, the cup springs back, increasing the size of the hollow and decreasing the pressure within. The circumference of the suction cup creates a seal that (for a reasonable length of time) maintains the lower pressure within the hollow.
#30
04-26-2012, 01:59 PM
 Irishman Guest Join Date: Dec 1999 Location: Houston, TX, USA Posts: 12,252
With a flat disk, air fills the available volume. The available volume is tiny, but air fills it.

With a suction cup, the available volume starts out larger, you squeeze the cup to flatten it and push air out, then the material tries to return to relaxed state. That return to relaxed state, increases the volume between the suction cup and the surface. The amount of air left in that volume finds a larger volume to occupy, and thus the pressure reduces. The balance point is the pressure difference against the material restorative force.
#31
04-26-2012, 02:09 PM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
Okay. So pressure is related to volume. It's obvious once it's pointed out.

So if one were to put one's mouth against a wall and suck, is it the volume increase in the lungs that will cause your mouth to stick to the wall?

At first thought, that doesn't sound right, but I guess if you think of the mouth and trachea and lungs as one cavity then the overall cavity increases in volume, thus decreasing in pressure.

But is the air pressing against the back of your head? If you could, theoretically, suck hard enough, would your head implode due to the pressure on it? I don't think the volume of your head has increased!

So what's holding you to the wall if you suck on it?

Last edited by davidm; 04-26-2012 at 02:11 PM.
#32
04-26-2012, 02:35 PM
 zut Charter Member Join Date: Apr 2000 Location: Detroit, MI Posts: 3,725
Quote:
 Originally Posted by davidm So if one were to put one's mouth against a wall and suck, is it the volume increase in the lungs that will cause your mouth to stick to the wall?
Yes. Or possibly just of your mouth: when I suck on a straw, I don't breathe inwards into my lungs, I flex my jaws and tongue.

Quote:
 Originally Posted by davidm But is the air pressing against the back of your head?
Well, yes. Why wouldn't it be? If you create a lower pressure area on one side of your head, then the pressure on the other side of your head is still there. And since it is, the pressure forces are not balanced, and an additional force is reuired, which is the conact force on your lips.
#33
04-26-2012, 02:56 PM
 Irishman Guest Join Date: Dec 1999 Location: Houston, TX, USA Posts: 12,252
Thank you. Thinking this through out loud is helpful.

Quote:
 But is the air pressing against the back of your head? If you could, theoretically, suck hard enough, would your head implode due to the pressure on it? I don't think the volume of your head has increased!
Theoretically, what pressure difference would be greater than the structural integrity of your skull? If you could suck that hard, you could implode your skull. Practically, I don't think it is possible.

Quote:
 Yes. Or possibly just of your mouth: when I suck on a straw, I don't breathe inwards into my lungs, I flex my jaws and tongue.
*Uses straw on desk* You are correct. I draw water into my mouth, flexing the tongue and jaw rather than lungs. This is actually important, as I don't wish to risk pulling water into my lungs, so I want to close off the air path to the lungs while trying to pull water into my mouth. Attempting it with my lungs open and it is actually a little scary, and a different process.
#34
04-26-2012, 03:01 PM
 davidm Charter Member Join Date: Mar 2002 Location: Near Philadelphia PA, USA Posts: 11,476
Okay then. So presumably we're not using our lungs to suck up a strand of spaghetti.

So, when we suck a drink, or spaghetti, are we creating a vacuum by changing the shape of our mouth somehow? You can suck for at least a few seconds and pull in quite a bit of liquid. All from suction created by changing the shape of your mouth?
#35
04-26-2012, 09:41 PM
 Irishman Guest Join Date: Dec 1999 Location: Houston, TX, USA Posts: 12,252
Yes.
#36
04-27-2012, 02:38 AM
 naita Guest Join Date: Jun 2002 Location: Norway Posts: 5,140
Quote:
 Originally Posted by davidm Think about a suction cup on a wall. You place it against the wall and push to remove the air. It then sticks to the wall because of atmospheric pressure. It sticks because there is no air (or a lower density of air) between it and the wall. Now take a round flat disk of the same material as the suction cup. Press it hard against the wall. Will it stick? No. Why? There's little or no air between it and the wall. There's atmospheric pressure against it. So what's the difference?
The difference is the suction cup is made of a malleable material and the suction cup design gives a force that fits the edge of the cup "perfectly" to the wall, preventing air from passing the seal, which means the force remains and increases friction so the cup doesn't slide.

You can get the same effects with a non-suction-cup design and zero air between surface and device, but it's more difficult to achieve while keeping friction. An example, with low friction is to take a smooth bottomed glass, place it on a smooth metal surface, like part of the kitchen sink, but wet one or both first. You now have a glass stuck to the sink, like a suction cup.

Your flat suction cup material disk won't work because if you press the disk against the wall and then release it, the forces in the disk will work to re-flatten it, letting air in at the edges and then further and further in. In the suction cup design the cup trying to reshape itself pushes the edges against the wall, keeping it fitting to the wall. With a softer disk you'll get a longer stick time.

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