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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).

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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 ($V\propto x^2$).

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  • $\begingroup$ I am not sure if SciMad is asking whether the object is floating or sinking. You definitely answer the first situation. Maybe you could also add one for last (the rotation will induce an inward acceleration, which will also mean an inward force due to buoyancy). $\endgroup$
    – fibonatic
    Commented Jan 31, 2014 at 9:03
  • $\begingroup$ My question was for sinking object (such as sand particles, undissolved sugar / salt). $\endgroup$
    – scimad.om
    Commented Apr 28, 2022 at 5:10

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