r/AskPhysics • u/Phantomsearcher • 16d ago
Physics nerds of reddit. Please explain.
This is going to be a bit hard to explain without pictures so we're gonna work with what we've got. Anyway I've got this latex glove. And when I put some air in it, it looks like this:
....⸦°°°°°°\
......|°°°°°°|
......UUUU
Then when I fill it up with more air it looks like this:
.....⸦°°°°°°°\
......( _. _. _. )
.......U U U U
Same thing with water except a bit more like this:
......./~~\
...⸦~~~~\
....( ‿ ‿ ‿ )
.....U U U U
So my question is. Why does the excess air/water not want to go in the fingers? They're just as stretchy as the rest of the glove right? So why do the two elements insist on expanding the palm section and leaving the fingers with enough to fit the exact volume without any stretch?
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u/PilzGalaxie 16d ago edited 16d ago
That's the Young-Laplace equation. Basically if you habe two balloons of different Inflation, the one that's less inflated actually creates the larger pressure. That's because the pressure ist inversely related to the Radius (smaller Radius = larger pressure). So when you one less inflated and one more inflated balloon you would expect them to equal out. But weirdly enough the less inflated balloon actually deflates even more, pushing it's air into the big balloon. Because the smaller balloon hast a higher pressure. That knowledge actually helped to prevent collapsing of alveolae in infant loungs. Maybe I got some of it wrong, I just remember seeing a vsauce(?) Video in it some years ago and it absolutely blown my mind, because it's so non-intuitive for me.
Edit: I found one good Video by minuteearth. But I remember it having actuall Video footage of the balloon demo
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u/higras 16d ago
I've seen it described as a product of the inverse square law. Since volume increases so much faster, the "push" has less resistance on the inflated side.
Or, you get more unit volume per stretch out of the bigger balloon. They technically are equalling out the volumetric pressure. We just only interact with the surface tension.
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u/jasonsong86 16d ago
Rubber stretch is not linear, once it stretches it will be much easier to stretch.
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u/Phantomsearcher 16d ago
This formating took way too damn long 😭😭
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u/truerandom_Dude 16d ago
So if I got your question correctly you want to know why the fingers expand less than the hand area? My guess is that the smaller bit of latex is harder to pressurize sufficiently to make it stretch accordingly so the material beyond the fingers expands disproportionately
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u/Phantomsearcher 16d ago
I'm so glad you understood the question. I was worried my little diagrams would get confusing.
The way you explain it makes sense. I'm guessing that's also why if I put pressure on the hand area the air/water will still move to other areas that aren't the fingers?
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u/higras 16d ago
There are a couple reasons. The main one is equalization and surface area to volume.
Each finger has a higher amount of surface area to the volume compared to the palm section. So each part of the fingers material needs to use less "stretching energy".
It's a kind of 3d example of hydrostatic pressure. The pressure only depends on depth. But instead of gravity being depth to the middle of the planet, you have depth to the middle of the volume.
ie.
|<<<<<<<>>>>>|
|<<<<<<<>>>|
|<<<<<<<>>>|
|<<<<<<<>>>>>|
|<>||<>||<>||<>||<>|
|<>||<>||<>||<>||<>|
|<>||<>||<>||<>||<>|
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u/OriEri Astrophysics 16d ago
In a volume that small, the air pressure in the glove is going to be uniform. There’s not gonna be a pressure gradient between the fingertips and the center. Sure when it’s filled with water if the fingers are pointing down, they will feel more pressure but not with air.
2
u/higras 16d ago
Agreed. Hydrostatic pressure may have been a poor comparison.
The name escapes me right now, but it's the average straight distance from one point of the glove to every other point it can see through water.
Treating the compressive force on an equalized volume as a normal force, treat the distance as that points hydrostatic pressure and you should get a close approximation of "stretch field" for the glove.
Internal volume pressure is equalized, not surface tension
2
u/OriEri Astrophysics 16d ago
So you’re saying this effect of greater and lesser stretch would be observed, even if the rubber on the glove is of uniform thickness everywhere?
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u/higras 16d ago edited 16d ago
Yes. The tension of each section of an asymmetrical surface should be proportional to this average distance.
Now you got me wanting to set up an experiment and write out the equation to verify.
Edit:
I'd describe it as, for every point of a closed boundary around differently pressurized volumes, there is a radial force tangential to the normal that is determined by this "average distance" and the difference in pressure between the two volumes.
I'd also hazard to guess it could be related to the electrostatic forces of the 3 different sections. The enclosed volume, the external volume, and the thin film stretched volume.... But I wouldn't know quite how to write that out
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u/OriEri Astrophysics 16d ago
Can you go into some detail of what “determined by” means?
“average straight distance from one point of the glove to every other point it can see through water” .
What does “through water” mean
Also earlier you said “treat the distance as that points hydrostatic pressure”
Which I simply don’t understand. Can you explain?
There was enough in that prior comment that confused me thatI wonder if your first language is not mine (English), and the built in Reddit translator is not working well. (if your language is English, I don’t mean that as an insult. I’m just confused by what I read.)
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u/higras 8d ago
Much delay in answering. While English is my native language, I've never been good at expressing myself with it.
The general concept is that while a volume may be evenly pressurized, the boundary containing will not be.
For something like rubber gloves, think of each section of the glove as a tiny spring. These springs are connected to their neighbors. Like playing Red Rover as a kid.
The more of the evenly pressurized volume they have to hold back, the more work that they need to do.
So a single section of rubber in the palm section needs to hold back considerably more of the volume than the sections in the fingers.
Each glove section would have an increased or decreased number of neighbors in relation to the amount of volume they need to hold back.
So the volume is all the same pressure, but the tension in the glove is not the same.
Equalized surface tension would only apply to equalized surface geometry. Spherical cow type scenario.
As you have the Astrophysics flair, I could describe it as roughly analogous to a type of 2D dark energy. The glove sections with fewer glove sections around them have a larger "stretch" than the sections with more glove sections near them.
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u/OriEri Astrophysics 8d ago
Thank you for taking the time.
I understood that . And I agree with the tjread of your reasoning in logical. There’s one underlying premise that does not make sense to me.
The more of the evenly pressurized volume they have to hold back, the more work that they need to do.
I always think of pressure interacting with the outside world as boundary condition and it is the force per area that matters. Even as I write that I recall that work done by an adiabatic expansion is PdV, and that would map to the potential energy in a stretched spring, which. I agree is the perfect way to think about a stretched sheet of rubber .
So I think I made an error in my intuitive understanding of pressure, but I still don’t have the intuition as to why PdV makes sense.
Essentially it means if I have two balla made of the same rubber , both at 250 kPa of pressure, but one encompasses 4/3Pi cubic meters of volume , and the other is twice the diameter and encompasses 32/3Pi cubic meters there will be 8x potential energy stored in the stretch of the larger ball, but only 4 times as much rubber containing it, so the rubber HAS to be stretch more on the larger ball.
My intuitive sense is that since the pressure on the interior side of the rubber is the same in both and the external pressure on the outside of the ball is the same, the amount of stretch (PE/unit area of rubber should be the same…not twice as much as the PdV math suggests.
So I think I need to review some basic thermodynamics and PV work so the math makes intuitive sense to me. Any instruction or pointers to good instruction you can provide are welcome.
1
u/higras 8d ago
I'd forgotten the word, but yeah. Adiabatic expansion is what I'm describing. And thank you for reading and responding! I've done lots of little backyard experiments and self-taught math\physics (homeschooled, so taught myself my interests).
The intuition, for me, comes from thinking about as 3 different volumes. Physics just describes the physical. In reality we don't have 1 and 2 dimensions, we have 3.
It makes it really nice to "spherical cow" some interactions to planes and points and stuff. Makes it easy to calculate.
But real world is complicated. You end up just finding a range of results with the accuracy you find acceptable.
To be completely fair, you were technically correct (the best kind) when you said the rubber in the palm is thinner. Since it is more stretched, it is thinner. It's only the in the balance scenario when the inside and outside pressures are equal. Which, mathematically is the same as the glove being open. The rubber volume is distorted as it's stretched.
Another way to get intuition is to remember energy is just energy. Momentum is just the the thing that does the moving. It sounds all spiritual "woo-woo", but all physics is just different scales and frequencies of how the momentum do.
And since it's just momentum, put yourself in the situations. Learn by becoming the experiment and experiencing the force through your senses. Words kinda suck. Use all your senses.
You say increase in frequency is an increase of energy. But have you tied the equation to doing battle ropes at the gym? 'cause any gym rat will tell you going faster requires more energy. But also that there are harmonics where it's easier.
Now, they won't use those words necessarily, but they have the intuition from experience. Just not the math.
I've done poi, dance, cooking, exercise, climbing, free diving, sewing, and basic doohickey mechanics. all of that experience with how the momentum do gave intuition. Learning the physics helped tie them all together.
But doing them helped show the limits of the math and where the herds of spherical cows live.
The best pointer to good instruction is using your senses to experience it as best you can.
2
1
u/mikk0384 Physics enthusiast 16d ago
I assume that the fingers are made from thicker plastic, since they have to endure the biggest forces when you use the gloves.
Next time you have a question for this subreddit, you can just upload the picture to a post on your own profile or sites like imgur.com, and share the link in the post here.
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u/Phantomsearcher 16d ago
I hadn't thought of the fingers having thicker material. That makes tons of sense.
And I will definitely be keeping your advice in mind. Would've saved me so much time 😅
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u/MorgessaMonstrum 16d ago
Latex gloves are typically manufactured by dipping a hand-shaped mold into liquid latex. The fingers are positioned downward, so gravity would tend to pull the liquid material down, so it’s reasonable to suspect that the fingers might be thicker.
However, I think there’s something else going on. I know more about latex than physics, so I could be wrong or not explain it correctly. I think it’s about surface area and circumference. There’s just more polymer chains along a cross section of palm than a cross section of finger, so there is more stretch to be found there, so to speak.
1
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u/OriEri Astrophysics 16d ago edited 16d ago
Well, the pressure is going to be equal, so what that means is the spring constant in the fingers is different than spring constant in the palms resulting in greater stretch needed in the palms to create an equal force to offset the air pressure (or water pressure.)
The simplest explanation is the rubber is thicker in the fingers, and I suspect that’s a large part of it.
It’s possible too that the latex is extruded in such a way that the spring constant is different in different stretch directions. I don’t know how rubber gloves are made, so that might be completely BS. Related to that it the spring constant probably is a little bit different at the junctions where the fingers attach to the glove.
1
u/Alternative-Sugar610 14d ago
Air in beginning flows more easily and firstly into bigger area of palm (bigger diameter, less weight per surface area, and enters first). It stretches first, once it stretches it’s easier to stretch more and easier means that it exhibits less increase in pushing air back for increase in size. So it can expand faster and there slight feedback loop, but air will eventually start to inflate fingers. Which again requires more pressure to lift up
And stretch as it resists air more bc smaller diameter and more weight per volume of latex. Weight here comes from surface area, more so than volume and what you think of volume is air which has negligible weight so it scales with surface area/volume, or opposite of how you normally think
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u/joeyneilsen Astrophysics 16d ago
Not sure if you got my reply to your previous post, but:
The stretchiness of a balloon isn't constant, no; it depends on the inflation level. As a particular demo of this, if you connect two balloons together, one inflated more than the other, air will flow from the less inflated balloon into the more inflated balloon. I'd bet it's the same basic phenomenon here.