1. Ok, shortly.
There is a wchichever object in water. We know that it will go up.
So why can't why we that power wchich is moving it for energy production?

2.

3. You can, but then it takes just as much energy to push it down into the water again, so you don't generate any net energy gain. If you had a large supply of buoyant objects under water you could extract energy from it, but it wouldn't be a renewable resource. Eventually you run out of buoyant objects and then no more energy.

4. OK, but why can't we do something like that
http://img5.imageshack.us/img5/3053/schemeou2.png

5. Moreover, because of inefficiencies in whatever system you are using to submerge the bouyant object, you need more energy to submerge it than you recover when it emerges.

6. Originally Posted by kordix
OK, but why can't we do something like that
http://img5.imageshack.us/img5/3053/schemeou2.png
Moving the ball into the water at the bottom of the trough takes exactly as much energy as pushing it down from the top of the trough.

7. OK, but what do you think about that?

////EDIT, aaa, now I see... we can do it only one time...
But, if we did f.e. 5 such holes?
And, in result-won't we get more energy than we will have used?

8. Originally Posted by kordix
OK, but what do you think about that?

////EDIT, aaa, now I see... we can do it only one time...
But, if we did f.e. 5 such holes?
And, in result-won't we get more energy than we will have used?
Yeah, it's a bit like that trick Daffy Duck did at the end of Show Biz Bugs.

You can set up as many stages as you want, but in each case it's a finite number of stages and so a finite amount of energy. More to the point, lifting the water high enough to get it above all the stages in the first place takes at least as much energy as you're going to get from releasing all your stages.

9. Originally Posted by kordix
OK, but what do you think about that?

////EDIT, aaa, now I see... we can do it only one time...
But, if we did f.e. 5 such holes?
And, in result-won't we get more energy than we will have used?
The energy in your cartoon comes from the gravitatinal field. In the first picture the water is above the bottom of the vessel and therefore has potential energy relative tot he state in the second picture. The bouyance of the ball results in some of the potential energy of the water being transferred to raising the ball against the gravitational field. But the loss in potential energy of the water is greater than the increase in potential when the ball is raised because the ball is lighter than an equal volume of water (which is why it floats). In the final configuration, the potential energy of the water has dissipated, some going to raising the ball and the remainder being realized as heat in the water.

So yes, you could extract some energy from your device, if you allow the ball to do some sort of work as it rises. But you can only do that one time. To repeat the process and return to the state of figure 1 you would need to apply external work to raise the water to that level.