Can You Make a Frictionless Loop Turn?

[Keasy]

TL;DR

If you have a hollow cylinder which is frictionless , non-magnetic and motionless, Is there any way to cause it to rotate about its longitudinal axis?

Suppose we are give a homogenous hollow cylinder or "normal" dimensions (for example 3" inside dia., 4" outside dia., 5" high". The composition is any non-exotic, non-magnetic, solid material. Is there any way to cause it to rotate about its vertical axis? Whatever the material is, any non-linear effects or surface deformation is avoided.

By frictionless we mean, as usual I think, any force we attempt to apply to the cylinder will only result in a force on the cylinder perpendicular to the vertical axis (z).

To simplify the question a bit while still retaining the key issue, the center of the cylinder is at the origin ox a x-y-z coordinate system with the z axis also the longitudinal axis of the cylinder. There can be no rotation of the cylinder about the x or y axis. Linear motion along the x, y, or z axis is fine.

To me it looks like the answer to this question is "no", but if I am wrong (not unheard of), there are some very interesting , very useful designs which are possible.

 

[berkeman]

I wonder if a rotating electric field (possibly non-uniform) could generate a torque...

 

[hutchphd]

How will you know the object is rotating? Use that measurement mode to make it rotate.

 

[Baluncore]

To me it looks like the answer to this question is "no", but if I am wrong (not unheard of), there are some very interesting , very useful designs which are possible.

Yes. If the tube was electrically conductive it could be spun like the rotor of an induction motor. That is how the rotating anodes inside glass X-ray tubes are driven.

Even if it was friction free, airflow past the cylinder would have a boundary layer, so it could be driven by tangential jets of air.
https://en.wikipedia.org/wiki/Coandă_effect#Mechanism

Are you planning on making a gas centrifuge, maybe to separate isotopes of uranium ?

 

[Keasy]

How will you know the object is rotating? Use that measurement mode to make it rotate.

I was thinking of sensitive optics to see if it is rotating. And of course in principle this could make it rotate, essentially using the momentum of photons. But this effect is so small its not really a useful solution.

 

[Keasy]

Yes. If the tube was electrically conductive it could be spun like the rotor of an induction motor. That is how the rotating anodes inside glass X-ray tubes are driven.

Even if it was friction free, airflow past the cylinder would have a boundary layer, so it could be driven by tangential jets of air.
https://en.wikipedia.org/wiki/Coandă_effect#Mechanism

Are you planning on making a gas centrifuge, maybe to separate isotopes of uranium ?

Thanks for those valid suggestions. I'm not looking at anything related to gas centrifuges; it is more related to understanding some force/torque configurations with some possibly useful configurations.

 

[hutchphd]

Yes. If the tube was electrically conductive it could be spun like the rotor of an induction motor. That is how the rotating anodes inside glass X-ray tubes are driven.

Interesting. Is there any way to tell it which way to turn?

 

[Baluncore]

Interesting. Is there any way to tell it which way to turn?

Yes.
A two or three phase rotating magnetic field outside the cylinder is created by using fixed stator coils. The rotating external field induces currents in the rotor, which then reacts by rotating with the field. You can also make an induction motor inside-out, with the fixed field inside a tubular rotor.

With a three phase supply there is a 120° phase shift between the phases. To reverse the sense of field rotation, swap any two of the three phase inputs.

Since you have very low friction in the rotor, there will be very low slip, so rotation will be close to synchronous with the supply frequency. What range of RPM do you require ?

https://en.wikipedia.org/wiki/Induction_motor#Principle_of_operation

 

[Keasy]

Just finding a way to cause the cylinder to rotate was my basic issue. I was thinking primarily of a set o torques or forces but this certainly is a more direct method.