Water inside bucket is rotated (by spoon or something) to flow in circular motion. An object kept in the bucket tends to be at the center of the bucket. Why is that?
Edit: The question is for sinking object (such as sand particles, undissolved sugar / salt).
In this answer here, I give a mathematical explanation why the surface of a rotation fluid is a parabola (or paraboloid, if you consider 3 dimensions). After you spin the fluid and the "parabola is formed" you drop the ball. If you don' have an external force to keep the ball on a given trajectory, after some time it will be located at the center. For a better understanding, look at the subject of stable equilibrium and the harmonic potential ($V\propto x^2$).
