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I have a wet teabag in an empty cup. If I will hold the teabag and touch the wall of cup with it, it will stick to the cup, like there would be glue or some magnetic field, but there's just water.

enter image description here

So, why does it happens? Why can just a little bit of water hold the teabag stronger than the gravity of an entire planet so it will not fall to the bottom of the cup, or even slide down a little bit?

Why is it so strong?

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7  
Electric force, or more indirectly, adhesive force of water. –  jhobbie Jul 17 at 17:39
5  
Because gravity is pathetically weak. –  ACuriousMind Jul 17 at 17:39
3  
Why is this being downvoted? It is a simple question but not asked badly. –  YungHummmma Jul 17 at 18:01
    
@ACuriousMind you kind of hit the essence of the question. It takes an entire planet to generate the energy needed to hold the bag in the bottom of the cup, but just a little bit of water have enough energy to defeat this force. So how can it be like this? Are the means of which gravitational force "travels" not efficient enough? Although we must consider the reach of the gravity, compared to the one in action on the bag which needs to touch to be effective... –  Bruno Finger Jul 17 at 20:29
1  
@Renan I think this demonstrates air pressure better. youtube.com/watch?v=Zz95_VvTxZM –  LDC3 Jul 18 at 2:05

3 Answers 3

up vote 10 down vote accepted

Based on your comment, I think you are indeed asking a more profund question than your teabag suggests:

Why is it that gravity is so weak compared to the other forces?

The answer is: We don't know. Seriously, that is one of the holy grails: To first find the Grand Unified Theory of nature in which all forces except gravity are explained as coming from one single symmetry group which is broken at our normal energies, and to then find the Theory of Everything that understands all four fundamental forces with a single, unified concept. In such a theory, it is hoped, the differences between the forces emerge naturally, instead of just being put in by measuring the relative strengths experimentally.

But we don't know yet. we don't know which of the manifold concepts that are at the forefront of theoretical thinking right now might turn out to be indeed the ones realized in nature. But the question why gravity is so weak is really still unsolved.

String theorists let gravity act in other dimensions, thereby weakening it in our four comparative to the others.

Others employ a form of the anthropic principle to argue that only in a universe where gravity is as weak as here life could have evolved and asked that question, so there is no deeper reason except that, for us to be here, it has to be like that.

I apologize to all those whose theory I cannot name and briefly, unjustifiedly compress into a few words off the top of my head.

That's it. We really don't know.

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1  
+1 for taking the question and answering it in a serious manner. I would like to give you an extra +1 for the part that says more profund question than your teabag suggests. –  Renan Jul 17 at 20:56
    
OK fine, we don't know why gravity is weak relative to other forces at this at distance, but isn't the answer really that's the way it is, and electrostatic forces explain the phenomenon? –  andy holaday Jul 18 at 1:58
    
@andy holaday: That answers the question: Why does the teabag stick to the cup?, not the question How can that sticky force be stronger than gravity?, which the OP, at least in my eyes, really asked in a clarifying comment. –  ACuriousMind Jul 18 at 2:03
    
Awesome answer, thanks for understanding what was really asked and providing such a good explanation. –  Bruno Finger Jul 22 at 15:03

The force you describe is a capillary force. The following diagram illustrates where this force comes from:

Capillary force

The diagram is supposed to show the thin film of water between the tea back and the wall of the mug. Any air/water interface has a surface tension that makes it behave like an elastic membrane. The air water surface is trying to shrink, and that means it is pulling the two surfaces together. This is obviously only a schematic diagram and the contact between the mug and teabag is obviously far more complicated than the diagram shows. However the same principle applies.

You ask why the force is stronger than gravity, but really all you can say is that the friction due to the capillary force is stronger than the gravitational force on the teabag. If you filled the teabag with lead shot instead of tea my guess is that the gravitational force would be greater and the tea bag wouldn't stick.

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Besides the adhesive force of water it might be good old friction, caused by the air pressure. As you mention the bag is wet. A vacuum could be created between the wall of the cup and the bag, causing the air to push the bag tight to the wall. This in turn causes a friction force between the wall and the bag.

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If that's so, why don't your hands get stuck together when you clap? –  Phil Frost Jul 18 at 11:55
    
@Phil Mine do, at least a little bit. –  zibadawa timmy Jul 18 at 12:35

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