# Object rotation in vacuum and the Friction force

1. Jul 16, 2011

### niralsoni

Any object rotating on the axis does stop because of some kind of friction force.

What happens if the same object rotates in the vacuum ?

does the friction force applies to it ?
or does it stop at any point in time ?
or does it continue to rotate forever ?

Thanks & Regards,
Niral Soni

2. Jul 16, 2011

### darkxponent

1) no friction force applies to it

2) it wont stop at any time

3) yes it will rotate forever

example:- earth, moon

3. Jul 16, 2011

### HallsofIvy

"In vacuum" does not necessarily mean no friction! If you have an object rotating around an axis and rotating around that axis, there will be friction between the object and the axis.

(Thanks for the correction.)

Last edited by a moderator: Jul 17, 2011
4. Jul 16, 2011

### disregardthat

If the object is not rigid it will cause stress due to the outward acceleration. I don't know if this necessarily will cause deformation and thus slowing the rotation, but I am quite certain that some mechanical energy will be lost because of this.

If the object is completely rigid, then special relativity has something to say about that: http://en.wikipedia.org/wiki/Ehrenfest_paradox

There are some problems with rigid bodies rotating because of length contraction. Apparantly a rigid body will immediately shatter if it is rotated, or so the article explains is one proposal.

Rigid rotating rods however, I think they would rotate forever.

The earth and moon not such a good example, since e.g. the moon cause tidal flow on earth which is a huge source of friction, and this does indeed slow rotation. Earth itself by rotating cause friction within the layers of the ground, and this is as well draining the mechanical energy of rotation.

5. Jul 16, 2011

### KingNothing

HoI...I think I see what you are saying, but this is a very, very strange way of saying it.

6. Jul 16, 2011

### Staff: Mentor

I think there was a typo and the last word should be "axis".

7. Jul 16, 2011

### niralsoni

thanks all of you for the quick reply.

what i understand from this discussion that there will be a friction force beteen the object and its axis, which in turn at some point in time will stop that object rotation.

please correct me if i am wrong.

Now, Can we relate vacuum with the zero gravity ?
what happens to the object rotating in outer space where there is a zero gravity?

And, say for example, in a vacuumed container, a rotating object is placed. Will there be any gravitational force applies to that object?

Thanks & Regards,
Niral Soni

8. Jul 16, 2011

### Staff: Mentor

A vacuum chamber does not eliminate gravity. They are not related concepts.

9. Jul 16, 2011

### niralsoni

ok..

10. Jul 17, 2011

### D H

Staff Emeritus
A torque is required to change angular momentum. For example, the Earth's rotation rate is gradually decreasing because the Moon exerts a torque on the Earth via the tides. Without no external torques acting on some object, that object's angular momentum will be conserved.

Angular velocity, however, is not a conserved quantity, nor is mechanical energy. A non-rigid but still cohesive body will eventually wind up rotating about the axis with the greatest moment of inertia.

11. Jul 17, 2011

### darkxponent

sorry i didnt get it....most objects in universe rotate around imagenry axis so how can there be friction between both...will u plz explain

12. Jul 17, 2011

### disregardthat

So would you say that a planet unaffected by external forces would rotate indefinitely, without regard to friction caused by the rotation itself?

13. Jul 17, 2011

### TobyC

If the object that is rotating is not perfectly symmetrical about its rotation axis then it will emit gravitational waves and lose energy that way. Neutron stars are expected to do this for example.

14. Jul 17, 2011

### Staff: Mentor

The original question said nothing about lack of gravity or lack of a supporting axis.

15. Jul 17, 2011

### darkxponent

than you sir for your reply. But this gives me one more doubt. Does this means that your saying that if perfectly symmetrical object rotates around itself will never loose its Energy and keep on rotating forever

16. Jul 17, 2011

### Staff: Mentor

If it is floating out in space and not connected to anything, yes. Conservation of energy demands that if nothing saps energy from it, it won't lose its rotational kinetic energy.

17. Jul 18, 2011

### darkxponent

oh thnx now i got it what you meant

18. Jul 18, 2011

### niralsoni

Considering the perfect symmetrical object rotating on its axis in vacuum, will it be possible to create a model arround it, here on the Earth ?

19. May 30, 2012

### vt1

If I put 2 bodies in a free falling vaccuum chamber, will they rotate around each other ?
Apologies if the question is dumb.