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when a disk is hung by a string attached to the center of mass point is tilted, what is making it fall and become horizontal again

Note the white line is a light string.

Here $Torque_{net} = 0$ and $F_{net} = 0$ so why does it rotate?

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    $\begingroup$ Does this really happen? $\endgroup$ Commented May 2, 2022 at 5:29
  • $\begingroup$ I don't think "hinged" is the word you want there. en.wikipedia.org/wiki/Hinge $\endgroup$
    – PM 2Ring
    Commented May 2, 2022 at 9:49
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    $\begingroup$ @CarlWitthoft I don't think so: physics.stackexchange.com/questions/13474/… The OP's scenario is not the same. See-saws on the play ground don't sit inherently balanced. I also believe the OP's scenario is not quite what they think it is. They may be thinking it is like a propeller balancer but the scenario described is dependent on how the string is connected to the plate and with most methods the string is bent when the plate tilts $\endgroup$
    – DKNguyen
    Commented May 2, 2022 at 13:37
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    $\begingroup$ @DKNguyen thanks for pointing out how it can differ $\endgroup$ Commented May 2, 2022 at 14:00
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    $\begingroup$ also related, physics.stackexchange.com/questions/247510/… $\endgroup$ Commented May 3, 2022 at 12:08

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Most methods of attachment bend the string when you tilt the plate at the point where the string connects to the plate. Strings under tension want to be straight. Along with non-zero plate thickness the COG moved laterally and up when the disc is tilted. So there are two things trying to level out the disc.

If the string were truly connected at the disc COG inside the plate and designed to not be bent when the disc was tilted, it would not level out.

For example, if a sphere was held captive in a near frictionless slot at the center of the disc and a string was connected to the sphere. This would just be a planar equivalent of a lever hanging from a ball bearing at its center, or a hanging see-saw, or a normal see saw.

This scenario is not the same as a balance scale which is designed to level out: Why does the weighing balance restore when tilted and released The balance scale scenario is similar to the non-zero thickness situation, but because of the string bending at the mounting point, your scenario would still result in a levelling out even if the disc were zero thickness.

Nor is your scenario the same as a see-saw or propeller balancer which are truly hinged and do not level out when balanced (which may have been what you thought your scenario was similar to).

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