Originally Posted By: EvilSOB


Once the pipe is long enough, the ball will pick up enough momentum during its 'fall' to push it across
the boundary between light and dense media.


Consider a square section and a cube , instead of a sphere , just to make it easier

The work made by the fluid is:

W = P * A * L

With P = density of the fluid * H

W = density of the fluid * V * H

The potential energy of the cube is :

U = density of the cube * V * H

If U > W Then density of the cube > density of the fluid

If the cube enters the pipe the fluid can not lift the cube