Hi to all, i will appreciate your help in this. This is the situation: I have a tank, with a water column in it. This tank at the bottom has an "ideal" seal (a seal that permit the passing of object from bottom to upwards, but not the water to fall down.) In this system i have sphere with density < water density, connected with cord. Click the link for images: https://www.dropbox.com/s/0k93koeqbmtt9ot/global1.jpg Bottom detail: https://www.dropbox.com/s/ls3go84h0oop3og/detail1.jpg I would like some explanation about forces and formulas. I have already written some, tell me your opinion. I have moreover the situations case, i would hear your opinion on these too: -CASE 1 (the second balance case is impossible because the opposing force to penetration for the ball in the bottom is always bigger than the buoyancy force of the other balls. Click for image: https://www.dropbox.com/s/8z2pc43tj7kj1yc/case1.jpg -CASE 2 (i have more balls out the water in the bottom side) https://www.dropbox.com/s/upyetwatlioagci/case2.jpg Note: LIMIT CASE: if i would have in place of all the submerged spheres a cylinder floater, with dimension: A:section, H hight (same hight of the tank), compleatly submerged, the buoyancy force of the cylinder will be the same of the opposing force at the bottom for the penetration of the ball ( density water*g*H*A). So neither in that limit case the ball will penetrate Thanks to all of you for your help and opinion!