[jeffreyH (OP)]

So that unless acted on by an external force an object would continue rotating with a constant angular velocity.

[RD]

... inertial rotation ...

Sounds like ... https://en.wikipedia.org/wiki/Angular_momentum

[Colin2B]

Sounds reasonable to me. Were you trying to relate it to a rotating inertial frame or just the rotational equiv of linear laws of motion.

[geordief]

Does any rotating object  change its rotation over time as a result of any  internal asymmetries ?

[Colin2B]

Does any rotating object  change its rotation over time as a result of any  internal asymmetries ?

Only if you force it to rotate on an asymetric axis ie one that isn’t through its centre of mass. If you are thinking of an unbalanced wheel, that would be forced to rotate on the wheel axis, but it wants to rotate on its cente of mass which is why it throws a wobbly.

[jeffreyH (OP)]

Sounds reasonable to me. Were you trying to relate it to a rotating inertial frame or just the rotational equiv of linear laws of motion.

It is a bit of both. I am a bit obsessed by inertia at the moment. I should get out more.

[Bill S]

  I should get out more.

Overcome your inertia.

[Colin2B]

I am a bit obsessed by inertia at the moment. I should get out more.

You mean go out on a roll?

[geordief]

Does any rotating object  change its rotation over time as a result of any  internal asymmetries ?

Only if you force it to rotate on an asymetric axis ie one that isn’t through its centre of mass. If you are thinking of an unbalanced wheel, that would be forced to rotate on the wheel axis, but it wants to rotate on its cente of mass which is why it throws a wobbly.

Well I was thinking about a body that was perfectly symmetrical (and so any axis of rotation could apply) .

I wondered whether ,given enough time internal asymmetries would develop as a result of internal interactions so that the body would emit radiation.

I was thinking in extremely large timescales and so maybe not relevant to the thread.

The body would not be rigid over large timescales ,would it?.

[yor_on]

Not sure I get it Jeffrey?

A inertial rotation? Are you thinking of Earth there? Earth is called a inertial frame but as it also rotates it's not 'perfectly inertial'.

https://en.wikipedia.org/wiki/Rotating_reference_frame

[Colin2B]

.I wondered whether ,given enough time internal asymmetries would develop as a result of internal interactions so that the body would emit radiation.....
..
...The body would not be rigid over large timescales ,would it?.

Not sure what you are thinking here, can you give an example of what sort of body you are thinking about and why it would emit radiation?
Any changes of mass distribution in a body should be distributed evenly around the centre of mass by centrifugal force.

[geordief]

I was thinking theoretically but what about a neutron star?

Would it be so homogeneous (made up entirely of the same objects ) that any redistribution would be entirely inconsequential?


A brown dwarf? Would they be practically spherical  except for a bulge around the "equator"?

The only object that would be completely symmetrical would be the BH  and the radiation I was thinking of could not escape but would there be asymmetries inside ?

Does the rotation of a BH  have any bearing on the distribution of  particles inside?

[Colin2B]

I was thinking theoretically but what about a neutron star?....
.....A brown dwarf? Would they be practically spherical  except for a bulge around the "equator"?

Most stars and planets are surprisingly symetrical so the mass does seem to be evenly distributed.

However, I think Jeff is thinking more of objects like flywheels

[jeffreyH (OP)]

Also it is about the moment of inertia. A rotating object moves with a range of velocities around its axis of rotation described by a perpendicular radius. It is like a collection of inertial frames. Time dilation depends partially on the distance along the perpendicular radius and partially on the objects gravitational field. The latter of which may be negligible.

[yor_on]

Inertial frames do not have a rotation

That's what's confusing me Jeffrey

[PmbPhy]

So that unless acted on by an external force an object would continue rotating with a constant angular velocity.

Yes. It's referred to as rotational inertia and can be calculated. Look it up in Wikipedia.

[PmbPhy]

I pulled out my old undergrad text by Haliday and Resnik and looked it up to be certain. It is called "rotational inertia" and represented by I just like mass is represented by M. The cool think is that they have similar expressions for kinetic energy. In general I is a tensor.  But in many cases it can be written using a scalar. In that case the rotational kinetic energy = (1/2)I*omega^2   where omega is the angular speed. Look familiar?  Sort of like K= (1/2) M*v^2