# zero rotational inertia

Discussion in 'Physics & Math' started by sciencerocks, Jan 28, 2006.

1. ### sciencerocksRegistered Member

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Let's say that something is spinning and it spins in a certian direction. The object has rotational inertia. Now say that the object has no inertia. How many times will the object change its direction of rotation in under a second?

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3. ### catoless hate, more scienceRegistered Senior Member

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how can something without mass/inertia change its direction of spin? it would have no mass to push against in order to be reversed. moreover, there are no outside forces defined which can change it.

I don't think it can be answered as it is a situation that, as far as I know, does not correspond to our known universe.

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5. ### DaleSpamTANSTAAFLRegistered Senior Member

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I agree with cato. I don't think that something without mass (aka inertia) can have angular momentum. Perhaps someone with some more QM knowledge could comment. I believe that photons have spin 0, do other massless particles have spin?

-Dale

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7. ### qwerty mobDeicidalRegistered Senior Member

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Make up your mind. =)

8. ### Physics MonkeySnow Monkey and PhysicistRegistered Senior Member

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As cato and others have noted, a classical mechanical system cannot have zero mass and a nonzero inertia tensor except perhaps in some singular limit. The inertia tensor is after all composed of moments of the mass distribution.

Also, photons are spin one objects.

9. ### superluminalI am MalcomRValued Senior Member

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And, correct me if I'm wrong, but when we talk about the "spin" of an elementary particle we are not implying that anything is actually spinnng (like a top or gyro), but that the particle exhibits properties that are analogous to angular momentum. Yes???

10. ### CANGASRegistered Senior Member

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There are actually too many peer reviewed books to count on quantum physics in which it is plainly stated that the concept of spin in nuclear particles, photons, etc. is an abstract concept and must not be considered to be equivalent to macro-sized spin of our everyday human experience.

11. ### DaleSpamTANSTAAFLRegistered Senior Member

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Really? I didn't know that. That's weird. They have angular momentum but no mass? What does that mean?

-Thanks
Dale

12. ### DaleSpamTANSTAAFLRegistered Senior Member

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My QM is very weak. I thought that the elemental particles had no size but still had angular momentum. Being a point the "spin" is not like the kind we are familiar with. But I thought it was still actually angular momentum.

-Dale

13. ### superluminalI am MalcomRValued Senior Member

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Dale,

Without digging hard for a definitive answer, I can't say. My QM is OK but the distinction between "actual" angular momentum and a property that is analogous to angular momentum is a subtlty that eludes me. Maybe a QM expert here can clarify?

14. ### CANGASRegistered Senior Member

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The elemental, or, fundamental particles protons and neutrons have been established by science experiments as having actual physical sizes. The electron, after a number of successive science experiments, each testing for a smaller possible size, has not yet been found to have a physical size. Perhaps even now another experiment is being conducted, testing for an even smaller size. Even though there are severe ramifications, it has been seriously proposed that perhaps the electron has no actual physical size and is a mathematical point.

I think it was Richard Feynman who said "Nobody understands quantum physics. We just learn how to get the right answers."

"Will a quantum physics expert stand up and explain this to us?"

What the h**l! How did my pants get stuck to my chair? Oh, well, I sure wasn't going to get up anyway.

Last edited: Jan 29, 2006
15. ### MacMRegistered Senior Member

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You have seemed to forget one possibility. If the device is tumbeling over its rotational axis it's direction of rotation reverses each 180 degrees of tumble.

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Last edited: Jan 29, 2006
16. ### superluminalI am MalcomRValued Senior Member

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Ha!

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17. ### Physics MonkeySnow Monkey and PhysicistRegistered Senior Member

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Quantum mechanical spin is indeed rather unlike the classical notion of spin. One can, for example, calculate the "radius" of the electron using the classical formula. Even assuming the "edge" of the electron moves at the maximum possible velocity, the result is a testable number which has been found experimentally to be much too large i.e. the experiments Cangas references.

That being said, spin is angular momentum in the quantum sense, and it is on equal footing with the more familiar orbital angular momentum. They both do all the things that you would expect angular momentum to do. The existence of spin is deeply connected with the structure of the fundamental interactions and the nature of spacetime. There is not, however, any understanding of spin in terms of more basic quantities. As such, it may be a fundamental component of nature, but we may never know.

18. ### superluminalI am MalcomRValued Senior Member

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"Nature is awesome in its every apparition and detail." - me