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I'm having a argument with a colleague, I don't know how to explain to him that if you spin a handspinner in space it will spin indefinitly (if you don't hold it). I agree that if you hold it, it will slow down because of the friction with the center part. Would it theoreticaly spin forever?

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  • $\begingroup$ @T.Auerrac Please use answers, not comments, to answer questions $\endgroup$
    – ACuriousMind
    May 12, 2017 at 12:07

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Assuming a perfect vacuum, you're both partially right. The spinner would slow down due to friction with the bearing. However, this also speeds up the bearing so at some point, the entire spinner is spinning at the same rate, at which rate it would spin forever.

Furthermore, even if you do hold it, the entire spinner will eventually stop spinning with respect to your hand, but will transfer angular momentum to you, so that you and the spinner are both spinning in space. If you then let go of the spinner it would continue spinning at the new lower rate, again indefinitely.

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Assuming that the engineering is perfect, that is, your handspinner can't be broken, that would be the case in the vacuum. It stops on Earth because of the friction with the air.

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Space is not a perfect vacuum, there are gases, electromagnetic fields and so on. The rotating object would be slowing down its rotation and in time, its motion will get erratic, driven by fluctuations of the environment.

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