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.
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.
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?
But air pressure in general is not irrelevant. Try your thought experiment in a vacuum; there will be no “suction.”
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.
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 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[sup]-8[/sup] to 10[sup]-7[/sup] 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.
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.
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.
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?
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.
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.
Thank you. Thinking this through out loud is helpful.
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.
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.
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?
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.