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Say you use your hand to rotate a string with a mass attached to the end. The string will first become taunt and then the mass and string will move upwards until they are spinning parallel to the ground. Why is this?

I know the string becomes taunt to stop the mass flying off at a tangent, but can't it do this at any angle? Gravity is pushing it down and I can't picture any other force in that direction to make it float up.

Why does it never rotate above parallel to the ground?

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closed as unclear what you're asking by John Rennie, ACuriousMind, user36790, Martin, Gert Apr 21 '16 at 0:07

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    $\begingroup$ The string will not eventually be parallel to the ground. At any speed there will be an upwards component of the tension force that balances gravity. $\endgroup$ – M. Enns Apr 18 '16 at 2:15
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    $\begingroup$ it is called "the conical pendulum" for the solutions see here en.wikipedia.org/wiki/Conical_pendulum $\endgroup$ – anna v Apr 18 '16 at 4:05
  • $\begingroup$ Draw the force vectors. $\endgroup$ – Carl Witthoft Apr 18 '16 at 11:29
  • $\begingroup$ In two words: centrifugal force. $\endgroup$ – Robin Ekman Apr 18 '16 at 13:15
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The rotation will not necessarily be parallel to the ground. The general motion will be a combination of rotation in a horizontal plane (the conical pendulum) and oscillation in a vertical plane (the simple pendulum).

If the support (pivot) is a fixed point, the motion you get depends on the starting conditions. If you launch the mass horizontally at the right speed, the motion will be a perfect horizontal circle. However, at any other speed the mass will oscillate vertically also.

If the support moves (eg oscillates up and down, or side to side, or in a horizontal circle), then it is possible to enhance the horizontal rotation while suppressing the vertical oscillation. (In fact, this is one way to get is started - a small up-and-down oscillation at the right frequency can turn into a large sideways swing of the bob.)

In either case, if the kinetic energy of horizontal rotational motion is very large, the vertical oscillation will be an insignificant wobble.

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