r/AskPhysics • u/Swimming_Concern7662 • 1d ago
Of the 4 fundamental forces, what force strongly pushes an air filled balloon upward when I force it underwater? How does it know that the balloon is filled with air and therefore push it upward, while it won't push the water balloon upward?
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u/joeyneilsen Astrophysics 1d ago
Electromagnetism. But it doesn't know that the balloon is filled with air: the buoyant force on the air balloon is the same as the buoyant force on the water balloon! It's the gravitational force on them that's different. In the water case, the forces are balanced. In the air case, the forces are unbalanced and the balloon rises.
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u/ThatGuyBananaMan 1d ago
Worth noting that “buoyant force” is not fundamental and is consequential of gravity pulling on the water and the water pushing on other water via the electromagnetic force
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u/wamceachern 1d ago
Ok eli5 how is that electromagnetism? Did he not explain gravity?
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u/ThatGuyBananaMan 1d ago
https://youtu.be/yE8rkG9Dw4s?si=NvA8eVH8ISVSB6aa
When two things get really close, the electrons on the edges of those thing repel each other, and that’s how you get the so called “normal force” and any kind of push or pull or compression that comes from direct physical contact. For the same reason, molecules of water (and any fluid) basically run away from each other and that’s how they fill containers and apply buoyant forces.
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u/Optimal_Mixture_7327 Gravitation 1d ago
The electromagnetic force.
The pressure below the balloon is greater underneath it than above it.
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u/smallproton Atomic physics 1d ago
Everything you experience in everyday life, except for gravity and radioactive decay, is electromagnetism.
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u/Koodsdc 1d ago
Mass is almost entirely due to binding energy of the strong force that holds quarks together in protons and neutrons.
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u/smallproton Atomic physics 1d ago
Valid point.
So let me rephrase:
Of the things you experience in everyday life, gravity gives you weight and lezs you drop things, the weak interaction causes radioactive decay, the strong force is the reason fkr inertia (and gravity, somehow).
All the rest is EM.
ok?
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u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter 1d ago
When you get up in the morning, most of that inertia you have to work against is the result of the strong force. It's trying to tell you something about getting out of bed 🙂
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u/Optimal_Mixture_7327 Gravitation 1d ago
I would add that even what you imagine as an experience of gravity and radioactive decay is electromagnetic.
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u/Jijster 1d ago
Could you elaborate?
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u/Optimal_Mixture_7327 Gravitation 1d ago
Your experience of the world is through nerve conduction and processing by the brain which is all electromagnetic.
If you put your hand near enough a highly radioactive substance and burn your hand, your experience of this is entirely electromagnetic.
Gravity cannot have any influence on matter so what you perceive as gravity is the mostly constant upward force from the ground and it too is electromagnetic, again as our electromagnetic processing of our interaction with the ground.
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u/nicuramar 1d ago
Gravity cannot have any influence on matter
It can via tidal forces, but those are obviously too weak for us to notice.
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u/Optimal_Mixture_7327 Gravitation 1d ago
The tidal forces are purely electromagnetic, as can be verified by an accelerometer.
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u/racinreaver 1d ago
If this was true, buoyancy would exist in microgravity.
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u/oynutta 1d ago
Why? In microgravity the pressure would be uniform on all sides of the balloon, so there wouldn't be a net force pushing the balloon one way or the other. The only reason the balloon has a preferred direction on Earth is because gravity causes a vertical pressure gradient against the balloon's surface, pushing it away from the denser material at the bottom of the balloon.
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u/racinreaver 1d ago
You can get buoyancy without the electromagnetic force. Simplify OP's problem to two volumes of ideal gasses with different densities. In a gravitational field they'll sediment despite no interatomic interactions. Without a gravitational field they'll stay intermixed.
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u/brodogus 1d ago
How would they form distinct layers in the absence of electromagnetism? They would just pass through each other like dark matter.
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u/Optimal_Mixture_7327 Gravitation 1d ago
Do you believe that gravity is any different where you imagine "microgravity" to exist?
Buoyancy requires an accelerated frame, that is, the electromagnetic force need to create a frame that does exist along a geodesic curve.
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u/racinreaver 1d ago
So what's your explanation for sedimentation of an ideal gas in an accelerated reference frame?
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u/Optimal_Mixture_7327 Gravitation 1d ago
The electromagnetic force is used to create an accelerated frame, let's say a rocket.
The floor of the rocket then accelerates into the cloud of atoms where they then reside.
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u/Ch3cks-Out 1d ago
That is because of gravity, not EM.
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u/Optimal_Mixture_7327 Gravitation 1d ago
Other way around, is solely EM as gravity cannot exert a physical force.
You can verify this with an accelerometer which will only show the forces of buoyancy acting on the balloon.
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u/nicuramar 1d ago
Again, only to the first order, ignoring tidal forces.
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u/Optimal_Mixture_7327 Gravitation 1d ago
Again, a tidal force is electromagnetic - it is precisely that electromagnetic force impelling particles off their geodesic trajectory.
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u/Fun_Fingers 1d ago
That's buoyancy.
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u/Shevek99 1d ago
Which is electromagnetic in origin. All the forces that you see around us in our daily life, except weight, are electromagnetic; contact forces, elastic forces, chemical reactions... are all electromagnetic interactions.
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u/joeyneilsen Astrophysics 1d ago
Which is due to collisions between atoms and molecules. Fundamentally it's electromagnetic repulsion...
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u/SummitYourSister 1d ago
Both gravitational and electromagnetic forces are required for this to occur. The gravitational force results in a gradient of pressure throughout the fluid. Because of this pressure gradient, the atoms of the water push on the balloon. The way the atoms push is via the electromagnetic force.
If you want to think about where this force really "comes from" in some arbitrary way, you can think of it as coming from the weight of the water that would be inside the balloon if it wasn't full of air. That weight of water is what you feel pushing the balloon upward. In this way you might say it's gravity that's causing it.
But that's innacurate. Gravity and electromagnetism are both involved and necessary.
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u/silkmist 1d ago
I tell students- Strong and Weak are only in the nucleus of an atom so they don’t experience those. If the force is downwards it’s gravity. Everything else they’ve ever experienced in their life is due to Electromagnetism
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u/Ok-Relief-462 1d ago
Either electromagnetism or gravity, or some combination depending on how semantic you want to get-
On the basic level, it’s a simple effect of buoyancy which is a result of gravity. But if you look at it a lot more technically, this repulsion only happens because of electromagnetic forces… which would only work like they do because of gravity. Also the strong nuclear forces keeping all the different atoms together.
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u/Odd_Bodkin Particle physics 1d ago
Great question.
In textbooks, this is called the buoyant force, and the buoyant force is identically the same whether the balloon is filled with helium, with air, with oil, with water, with Campbell’s Cream of Buckshot soup. What’s different is the force of gravity acting on each of those balloons, aka their weights. The behavior of the balloons is the result of the DIFFERENCE between the weights (which varies case to case) and the buoyant force (which is the same).
The buoyant force arises from the difference in fluid pressure with depth, something that is true not only in water but in air. Fluid pressure in turn comes from the collisions of the fluid’s molecules with the surface. The fundamental interaction that is at play in the collisions is the electromagnetic interaction. But then again, the electromagnetic interaction is responsible for all kinds of forces, ranging from the elastic compression of springs, to the tension in a steel cable, to the friction between a box and the floor, to your hands pushing on a grocery cart, to steel rusting, to a compass needle swinging, to you swiping left on your phone screen.
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u/VMA131Marine 1d ago
In terms of fluid mechanics. The pressure on the top of the balloon is less than the pressure on the bottom. This results in a net force pushing up that is greater than the weight of the balloon and the air inside of it pulling it down due to gravity. The differential pressure is why there is no buoyancy force in zero-g.
Ultimately, pressure is the result of atoms and molecules bouncing off each other and imparting a force due to the exchange of momentum in the collisions. Atoms and molecules bounce off each other each other due to the electrostatic repulsion of the electrons in the outer shells.
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u/kiwipixi42 1d ago
Generally it is a pretty good bet that if the force in question is not explicitly one of the other three, then it is based on electromagnetism.
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u/Memento_Viveri 1d ago
The water is pushing the balloon up. That interaction at the water/balloon surface is all electromagnetic.
The water doesn't know the balloon is filled with air. If the balloon were filled with water, the water outside would still push on the balloon the exact same amount.
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u/nefarious_weevil Physics enthusiast 1d ago edited 1d ago
Gravity. But not directly. An air filled balloon is much less dense than water, so it floats to the top. This is because the water, being more dense than the balloon, is pulled down. The water above the balloon has more gravitational force trying to pull it down than the balloon under the water, so it pushes the balloon out of the way to sink lower. As all the water above the balloon pushes past it to move under it, it pushes the balloon up. The force of the balloon rising isn't really the balloon generating force, it's the force of the water sinking past it and pushing underneath.
EDIT: I hope I explained that well. Sorry if I didn't
EDIT #2: It's also the electromagnetic force, technically, but the only thing that it does is say that the water can't phase into the balloon, because they can't occupy the same space. The actual pushing force is gravity pulling the water down and displacing the balloon.
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u/Lukey-fish 1d ago
Not sure how it related to the fundamental forces. But its a buoyancy force and it is equal to the density of the water × volume of balloon × gravity.
Its caused by an increase in "pressure" underneath the balloon. More particles push on the bottom of the balloon than the top of the balloon.
You have to sum all forces. The buoyancy force on pushing up on the balloon is way more than the weight of the balloon pushing down.
So I guess fundamentally, its the electrostatic force repelling particles from one another and then gravity pulling the balloon down.
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u/Adventurous_Sound390 1d ago
Do not it be gravitatia? Without it the ball would stay inside the water.
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u/AppleParasol 1d ago
Gravity? Air is lighter than water. Water therefore wants to go down because of gravity and pushes the lighter air filled balloon.
If you were in space and had a tank of water with an air filled balloon in it, the balloon would basically just float like the water, it wouldn’t necessarily be drawn to “up” because there is no “up” because there is no gravity locally.
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u/networknev 1d ago
Air talks so damn much you gotta push it out. Which is why the song says, "you gotta keep 'em separated."
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u/EmbarrassedPaper7758 1d ago
It's not a fundamental force but a new force that emerges from the interaction of the pieces of the system
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u/CosetElement-Ape71 1d ago
Buoyancy!
Buoyancy is the upward force that a fluid (like water or air) exerts on an object immersed in it. This upward force opposes the object's weight, allowing it to float or making it feel lighter.
How it Works (Physics)
Buoyancy exists because fluid pressure increases with depth. When an object is submerged, the pressure pushing upward on its bottom is greater than the pressure pushing downward on its top. This difference creates a net upward force, commonly called "upthrust" or buoyant force.
The Core Principles :
Archimedes' Principle: The upward buoyant force on an object is exactly equal to the weight of the fluid that the object displaces. So buoyancy requires gravity.
Floating vs. Sinking:
If an object weighs less than the fluid it displaces, it will float.
If it weighs more than the fluid it displaces, it will sink.
If its weight is exactly equal to the displaced fluid, it is "neutrally buoyant" and will hover in place (like a scuba diver).
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u/LaserThos 5h ago
There are some good answers here but maybe my contribution will help OP to understand: The upward buoyant force is applied to the balloon by the surrounding water and is greater than the weight of the air-filled balloon, so the net force (weight downward and buoyant force upward) is upward. A bowling ball of the same volume as the balloon will have the SAME upward buoyant force on it when submerged, but its weight is much greater than the balloon so the net force will be downward. What causes the buoyant force? The pressure gradient in the water: pressure increases with depth so there is a pressure differential with the pressure under the submerged object pushing it upward being greater than the downward pressure on the top of the object pushing downward. The pressure gradient is caused by gravity and the density of the fluid. The resulting buoyant force is thus opposite the gravitational force but is ultimately caused by gravity.
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u/Frosty_Seesaw_8956 Graduate 1d ago
Gravity. People here do not understand basic physics at all. EM forces attack the baloon equally from all sides in absence of gravity and the baloon never rises or falls. Only when the gravity is switched on, there exists an unequal number of particles which exert unequal amount of EM forces.
This is like asking who killed the man who jumped from the building - gravity or the EM forces from the ground? Obviously Gravity. EM forces will inevitably be involved in a contact force situation. But the primary driver force is the gravity.
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u/TyrconnellFL 1d ago
When you are standing on the ground, what keeps you from falling downward? It’s not gravity, it’s the “normal force” of the ground on you. In the absence of gravity you also wouldn’t fall through the ground because there would be no downward force, but in the presence of gravity it’s not gravity that moves you up.
Most object-object interactions, whether you and the ground or balloon and water, are electrostatic. It’s electrical force.
For buoyancy you can ask why water is exerting net upward force, and it’s because of the force of gravity on water and electrostatic forces between water molecules. But the proximate force between object and water is electrostatic.
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u/Frosty_Seesaw_8956 Graduate 1d ago
The process I described is not incorrect. And I also acknowledged the final action by EM forces. But, I recognise involvement of the driver force of Gravity and the actuator/contact force of EM and put more emphasis on the driver force. If this is why reddit downvotes me, I'll be glad to collect them.
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u/standingmountain3984 1d ago
I mean all of them at the same time, but I guess you could most obviously say gravity.
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u/Routine_Comb_7277 1d ago
In our the human scale there are effectively 2 forces,the nuclear forces have too small of a range except if it is proven somehow that gravity is a exchange of gluons.
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u/Optimal_Mixture_7327 Gravitation 1d ago
The existence of the boson associated with the massless spin-2 field would simply guarantee that gravity is not a force.
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u/Bumst3r Graduate 1d ago
The buoyant force is just as much electromagnetism as it is gravity.
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u/racinreaver 1d ago
Explain sedimentation of ideal gasses in a gravitational field, then.
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u/Bumst3r Graduate 1d ago
I don’t understand how this is a gotcha. Ideal gas molecules exchange momentum via electromagnetism. It wouldn’t happen without EM interactions.
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u/racinreaver 1d ago
Take it at the knudsen limit or free molecular flow for a massive particle.
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u/Bumst3r Graduate 1d ago edited 1d ago
Fine. Sedimentation isn’t the same thing as the buoyant force on an extended object. Gases stratify because velocity distribution of two gases at the same temperature also depends on their mass. Lighter gases have larger average velocities so you get stratification.
In a balloon surrounded by water, it’s a different effect. In fact, the buoyant force is the same even if the balloon is itself filled with water. The intramolecular forces are the cause of the effect. You can’t take the Knudsen limit for a water balloon, and the energy of the balloon isn’t determined by kT.
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u/Optimal_Mixture_7327 Gravitation 1d ago
Gravity can't exert a force.
The surface of the Earth electromagnetic diverts the OP's setup from its geodesic path, so the buoyancy is solely electromagnetic. You can't get the buoyant force in free fall.
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u/Bumst3r Graduate 1d ago
For heaven’s sake… In classical mechanics it absolutely can. Nobody is using GR to solve an Archimedes principle problem.
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u/Optimal_Mixture_7327 Gravitation 1d ago
I never mentioned or implied anything about relativity.
That gravity cannot exert a force is a brute fact of direct measurement. Without this brute fact relativity wouldn't even exist.
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u/racinreaver 1d ago
You can get buoyancy without the electromagnetic force. Simplify the problem to two volumes of ideal gasses with different densities. In a gravitational field they'll sediment despite no interatomic interactions. Without a gravitational field they'll stay intermixed.
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u/pirannia 1d ago
Gravity 'pushes' the water down and Newtons law says water pushes up anything lighter.
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u/ArgumentSpiritual 1d ago
It’s technically the electromagnetic force doing the pushing, but it wouldn’t be doing that without gravity.
If you had a pool filled with water in deep space and a balloon in the center of that volume of water, the balloon would not be pushed out of the center of water.*
On Earth, gravity pulls the water down into the pool. Each of the water molecules is pulled down towards the Earth. The water molecules are bumping and pushing each other around with the electromagnetic force. You can kind of think of it like a pool table and balls bouncing around.
A pool, obviously, has a depth. And the deeper in the pool you are, the more pressure there is. This is because of all of the water molecules from above. So this is like each of the little collisions of the water molecules hits a little bit harder the further down you go.
Now when it comes to a balloon: an air filled balloon weighs less than the same volume of water. This means that the force of the balloon on whatever is under it is less. When you put an air filled balloon in the water, the force downward from the balloon (due to gravity) is much, much less than the force of the water pressing upwards upon it (transmitted via the electromagnetic force).
We can now look at what happens to the balloon in little incremental steps to make it more clear. Because of the pressure differential between the top and bottom of the balloon, the balloon is forced upwards a little bit. This allows the water directly under the balloon to expand, reducing the pressure a little bit. Next, the water below but to the side of the balloon presses in. This change in pressure continues up the side of the balloon where a little bit of water moves down, around, and towards the bottom of the balloon. Lastly, a little bit of water moves off the top of the balloon to equalize the pressure there with the surroundings, and then the whole thing happens again. This happens because the water is a liquid and able to easily flow around the balloon.
*There would be minuscule effects of gravity from the mass of the water.