Sum of forces, vacuum and gravity

[Gh778]

It's a theoretical study. I would like to understand how the sum of forces can be at 0 if I put an object (vacuum in it) in a big liquid disk (disk is fulled with liquid), the disk is big enough for agglomerate liquid (like this works with a planet, matter is agglomarate with gravity). There is no external circular wall for disk ! it's very important. I attach the vacuum object with something external at disk (black color). The vacuum object has an extenral force give by liquid, force of buoyancy, this force is transmited to black object. I study the sum of forces on all the system (vacuum object+disk+black object). What's compensate this force ? How liquid can compensate this force ? And if liquid compensate the force, this would say if I put a bubble in water the bubble go to one direction and water go to the contrary direction, this would change the center of gravity.

 

[A.T.]

Do you mean the force of buoyancy on the vacuum bubble?
http://en.wikipedia.org/wiki/Buoyancy

 

[Gh778]

yes, the force of buoyancy

 

[A.T.]

And if liquid compensate the force, this would say if I put a bubble in water the bubble go to one direction and water go to the contrary direction, this would change the center of gravity.

Center of gravity of the whole system will not move. Fluid will move one way, the vacuum box and whatever is attached to it will move the other way.

 

[Gh778]

So, if you're right: imagine a fluid disk (or a sphere) without black solid. I put vacuum object in water, this don't change the CG. After, I let the bubble move, this move water in one direction and vacuum object in other direction, but the vacuum object has no weight, this move the CG ?

 

[mfb]

Even if your vacuum bubble has no mass, the fluid has. The vacuum bubble will move outwards, and the center of the fluid will be exactly at the position where the initial center of mass was. While the fluid moves around your bubble, the remaining fluid will move, too.