Rotation and gravitational potential in relativity.

[koti]

15 minutes ago, swansont said:

It should. Meanwhile, translation will not. The key here is, as you note, energy is added to the object, and it remains in the same frame of reference. The mass must increase.

Right. My gripe was with rotation, it somehow eluded me that as opposed to translational motion, rotation will add energy to the frame. Which is obvious because the energy for adding the rotation has to come from the outside. 

So simple...how could I not take into account that the energy has to come from outside the frame.

 

Edited March 18, 2018 by koti

[swansont]

Just now, koti said:

Right. My gripe was with rotation, it somehow eluded me that as opposed to translational motion, rotation will add energy to the frame. Which is obvious because the force for adding the rotation has to come from the outside. 

So simple...how could I not take into account that the force had to come from outside the frame.

The force can come from inside the frame, but for translation it means there are two frames. 

[koti]

3 minutes ago, swansont said:

The force can come from inside the frame, but for translation it means there are two frames. 

Ofcourse, its so clear now. 

(I edited force into energy for simplicity)

[J.C.MacSwell]

Also of course, if the energy comes from inside the ball/system there will be no net change of energy and no increase in gravitational potential or inertia.

[koti]

3 minutes ago, J.C.MacSwell said:

Also of course, if the energy comes from inside the ball/system there will be no net change of energy and no increase in gravitational potential or inertia.

Yes, that is the whole point of my embarrassment. I fell for the oldest trick in physics :/ 

[swansont]

45 minutes ago, J.C.MacSwell said:

Also of course, if the energy comes from inside the ball/system there will be no net change of energy and no increase in gravitational potential or inertia.

Not sure how that would happen, since the angular momentum must come from something outside the system, exerting a torque on it. Which, if the torque causes a rotation, must involve work being done.

[koti]

28 minutes ago, swansont said:

Not sure how that would happen, since the angular momentum must come from something outside the system, exerting a torque on it. Which, if the torque causes a rotation, must involve work being done.

I think what J.C MacSwell means is that if the ball had say a motor a battery and some kind of gyroscopic rotational propulsion system that would provide the angular momentum and resulting torque from within the frame, the net energy has to alwyas account for - in this case it would be potencial energy stored in the battery within the same frame as the ball which would not cause an increase in gravitational potential in that frame.

[swansont]

39 minutes ago, koti said:

I think what J.C MacSwell means is that if the ball had say a motor a battery and some kind of gyroscopic rotational propulsion system that would provide the angular momentum and resulting torque from within the frame, the net energy has to alwyas account for - in this case it would be potencial energy stored in the battery within the same frame as the ball which would not cause an increase in gravitational potential in that frame.

 Fair enough. You could also heat the ball up. Internal source means no change in energy. External source, and mass increases.

[J.C.MacSwell]

3 hours ago, swansont said:

Not sure how that would happen, since the angular momentum must come from something outside the system, exerting a torque on it. Which, if the torque causes a rotation, must involve work being done.

If say a very large mass (ideally infinite) was used outside the system to provide the angular momentum, the energy could come from within the system. This would provide the angular momentum  without the energy coming from outside. The mass and gravitational potential would therefore not change.

3 hours ago, koti said:

I think what J.C MacSwell means is that if the ball had say a motor a battery and some kind of gyroscopic rotational propulsion system that would provide the angular momentum and resulting torque from within the frame, the net energy has to alwyas account for - in this case it would be potencial energy stored in the battery within the same frame as the ball which would not cause an increase in gravitational potential in that frame.

Unfortunately you cannot get the system/ball to actually spin if you mean this to come from within the system/ball . You could get the appearance of spinning but no change in angular motion from within. This is different from what I had suggested earlier with the energy coming from within, but by necessity the system/ball has to be open to external forces for the reason Swansont mentioned

Have to be a little careful describing what is within a system vs what is within a frame, and  the context implied or assumed 

[koti]

1 hour ago, J.C.MacSwell said:

Unfortunately you cannot get the system/ball to actually spin if you mean this to come from within the system/ball . You could get the appearance of spinning but no change in angular motion from within. 

I’m not sure I understand. If potential energy from a battery-mechanism inside the ball is released and it becomes rotation of the ball, isn’t that transformation from potential energy into angular motion from within the frame? Obviously not causing any change in mass/energy/gravitational potential within the frame. What do you mean by appearance of spinning? 

[J.C.MacSwell]

28 minutes ago, koti said:

I’m not sure I understand. If potential energy from a battery-mechanism inside the ball is released and it becomes rotation of the ball, isn’t that transformation from potential energy into angular motion from within the frame? Obviously not causing any change in mass/energy/gravitational potential within the frame. What do you mean by appearance of spinning? 

Hi Koti

Sorry. I meant to type angular momentum where I typed angular motion.

You can't change the angular momentum of an isolated system. What I meant by the appearance of spinning is, for instance, you could spin the surface of a ball one way with the inside spinning the opposite, giving the appearance that the ball itself is spinning with the ball/system having no net spin.

 

[koti]

27 minutes ago, J.C.MacSwell said:

Hi Koti

Sorry. I meant to type angular momentum where I typed angular motion.

You can't change the angular momentum of an isolated system. What I meant by the appearance of spinning is, for instance, you could spin the surface of a ball one way with the inside spinning the opposite, giving the appearance that the ball itself is spinning with the ball/system having no net spin.

 

No worries, I presumed it was a typo, I was just trying to make sure as I’m just a „hobbyist physicist” and things can get confusing for me sometimes. 

Yes, angular momentum is a conserved quantity and cannot be changed unless acted uppon by an outside force - I knew that one  

That hidden zero net spin on the ball due to layers revolving opposite directions would be a nasty case. Good thing this is not the case.

Edited March 19, 2018 by koti

[Bender]

The rotational energy could increase if the gravitational potential energy decreases (without changing the angular momentum obviously ) . I have not found a conclusive answer to the question how much gravitational potential energy itself adds to the mass/energy.

Trouble is that there is no zero reference for potential energy. If you assume an object has the potential to collapse into a singularity, its potential energy becomes infinite. Does anyone have a solution to that?  

[koti]

On 19.03.2018 at 10:24 PM, Bender said:

Trouble is that there is no zero reference for potential energy. If you assume an object has the potential to collapse into a singularity, its potential energy becomes infinite. Does anyone have a solution to that?  

I don’t think this is right Bender. As far as I know, potential energy or any other energy/mass/momentum or tensors associated with energy, cannot reach an infinite value. The ball rotation velocity is always limited by c so in my thought experiment, if a ball would be to rotate itself into a black hole it would have to have mass/energy right on the edge of colapse, the energy due to the rotation would be very small. I don’t know how small exactly, I found the Kerr metric but I’m not sure it can be applied here (before the colapse)