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I live in a very tall building so I thought it would be interesting to see when my building is swaying by setting up a pendulum.

It works great, but I have noticed that over time the tip of the plumb-bob becomes loose. I have screwed it back in several times (but not tight enough to "stick"), and over time it becomes more and more unscrewed, as seen in the picture.

Is this is result of torque applied to the plumb-bob via the rotation of the Earth?

A diagram of my plumb-bob

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  • $\begingroup$ It is far more likely that it is a local effect. Is the motion of the pendulum perfectly smooth? Have you tried just leaving the bob on a shelf for the same period of time and seeing if it loosens? $\endgroup$ – TimWescott Dec 5 '19 at 23:14
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It looks like thread self-loosening can occur when a bolted joint is subjected to dynamic axial load (see article "Study on self-loosening of bolted joints excited by dynamic axial load", Tribology International 115 (2017) 432-451), and you have such axial load varying with time in your pendulum.

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