No matter how fast you swing the mass, the angle that the mass was swinging below the horizontal would never reach zero degrees (i. e., it would never swing perfectly in the horizontal plane). Why

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As the stopper is been swung in a horizontal plane the Tension force is balanced by a centripetal force and not a vertical force, moreover there is a downward force(mass of the stopper) acting on the stopper and there is no corresponding upward force, hence the angle that would be made by the swinging of the mass no matter how fast  would be : ∅ > 0

Explanation:

No matter how fast you swing the mass the angle will not reach zero degrees and this is because

As the stopper is been swung in a horizontal plane the Tension force is balanced by a centripetal force and not a vertical force, moreover there is a downward force(mass of the stopper) acting on the stopper and there is no corresponding upward force, hence the angle that would be made by the swinging of the mass no matter how fast  would be : ∅ > 0

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