I have a tank with water (10 m high) , with an ideal seal at the bottom (water can't fall down, but can enter bodies).
I have a system of 6 cubes ( of polystyrene density= 20 Kg/m^3) with dimension 1x1x1 m. These cubes are connected with a rope (volume negligible). They are in vertical column, and are all submerged except one, that is out in the bottom part.
So there is a buoyancy force (the 5 cubes) that will pull up, the weight forces of the cubes (rope volume and weight negligible) that go downwards, and the opposing force at the bottom, in the seal for the cube that is out. This force is caused for the column of water.
The result of all this forces doesn't allow the bottom cube to penetrate completely (case impossible). Is the buoyancy force sufficient to lift the bottom cube into the water, given that the column of water is pressing down on the seal? If the cube does get pulled up, then how far?
Best Answer
No, the buoyancy of the upper cubes can never be enough to even begin to pull the bottom cube into the water.
Note that only the pressures on the top and bottom surfaces of the cubes are relevant. For each cube in the water, the difference in this pressure is related to the height of the cube, since each cube has the same horizontal crossection. The total buoyancy force from the string of cubes is therefore proportional to the total height of all the floating cubes. However, the downward pressure on the top face of the cube trying to enter at the bottom is the full water column height. This is clearly more than the total height of all the cubes, and can never be less than the height of all the floating cubes.
This is one of the "free energy" concepts that pop up regularly. Usually they have it "almost working", just need funding to perfect the bottom seal. It seems there will always be a supply of people that didn't pay attention in physics class.