Does the rotation speed of a planet affect its gravitational pull

[eiyaz]

For example let's a satellite was orbiting the moon which has no atmosphere. If the moon suddenly started spinning twice as fast would it effect the satellite's orbit even though the satellite is separated by the vacuum of space from the moon?

Easier way to put it, if the Earth suddenly spun 1000 times as fast, would the moon float away? I know that things on the planet may be thrown off due to centrifugal force, but would the Earth's gravitational pull become weaker?

 

[Comeback City]

I believe it would, since angular momentum needs to be conserved in such a system.

 

[PeroK]

For example let's a satellite was orbiting the moon which has no atmosphere. If the moon suddenly started spinning twice as fast would it effect the satellite's orbit even though the satellite is separated by the vacuum of space from the moon?

Easier way to put it, if the Earth suddenly spun 1000 times as fast, would the moon float away? I know that things on the planet may be thrown off due to centrifugal force, but would the Earth's gravitational pull become weaker?

The force we identify as gravity here in Earth is, in fact, slightly less than it would be if the Earth were not spinning. The full force of gravity is offset to a small amount by the centrifugal force. Or, more precisely, some of the gravity is used as a centripetal force to keep us rotating with the Earth.

This, however, does not affect objects in orbit. The moon's orbit is essentially unaffected by the Earth's spin.

 

[eiyaz]

I believe it would, since angular momentum needs to be conserved in such a system.

Why does angular momentum need to be conserved? Centrifugal force is fictitious force when means its not mediated by a particle. If an object is floating a in circular space station in a vacuum and the space station is spinning creating a centrifugal pulling everything down at 9.8 m/s^2, but the floating object will float forever unless it is attached to the station and grabbed by its spin.

 

[eiyaz]

The force we identify as gravity here in Earth is, in fact, slightly less than it would be if the Earth were not spinning. The full force of gravity is offset to a small amount by the centrifugal force. Or, more precisely, some of the gravity is used as a centripetal force to keep us rotating with the Earth.

This, however, does not affect objects in orbit. The moon's orbit is essentially unaffected by the Earth's spin.

So a spinning black hole should not lose its event horizon no matter how fast it spins?

https://astroengine.com/2009/08/09/the-naked-singularity-recipe-spin-a-black-hole-add-mass/
Is this article wrong?

 

[Janus]

So a spinning black hole should not lose its event horizon no matter how fast it spins?

https://astroengine.com/2009/08/09/the-naked-singularity-recipe-spin-a-black-hole-add-mass/
Is this article wrong?

Spinning black holes are unique animals. For example, they have two "surfaces", the event horizon and the "ergosurface". Anything that enters the region between these two is forced to orbit with the black hole's rortation due to frame dragging( its impossible for an object to orbit in a direction opposite to the rotation).
The article says nothing about the gravity of the black hole weakening, but instead mentions a theoretical method of "disrupting" the event horizon. (A weakening of the gravity would just lead to the event horizon shrinking.)

The Earth also does some frame dragging, however it is very very weak, and even spinning the Earth up by 1000 times wouldn't cause a significant increase in it.

 

[Nugatory]

So a spinning black hole should not lose its event horizon no matter how fast it spins?

You started this thread asking about the rotational speed of a planet, not a black hole. The gravitational effects of rotation are altogether negligible for a planet, which we can properly analyze using ordinary Newtonian theory.

 

[PeroK]

So a spinning black hole should not lose its event horizon no matter how fast it spins?

https://astroengine.com/2009/08/09/the-naked-singularity-recipe-spin-a-black-hole-add-mass/
Is this article wrong?

I had a bad feeling that a black hole was lurking behind your initially innocuous question!

 

[DrStupid]

The force we identify as gravity here in Earth is, in fact, slightly less than it would be if the Earth were not spinning.

How about the poles?

 

[Comeback City]

Why does angular momentum need to be conserved? Centrifugal force is fictitious force when means its not mediated by a particle. If an object is floating a in circular space station in a vacuum and the space station is spinning creating a centrifugal pulling everything down at 9.8 m/s^2, but the floating object will float forever unless it is attached to the station and grabbed by its spin.

Law of Conservation of Angular Momentum

 

[Khashishi]

Conservation of angular momentum has nothing to do with the question that the OP is asking.

 

[Comeback City]

Conservation of angular momentum has nothing to do with the question that the OP is asking.

What, is Conservation of Angular Momentum not a thing anymore? Or has George Gamow been teaching me false information...

 

[pixel]

With Newtonian gravity, if the planet around which the satellite is orbiting is spherically symmetric then rotation doesn't matter as the satellite "sees" the same gravitational source at all times. If it's not spherically symmetric, then there would be perturbations to the satellite's orbit and those would be different for different rotation speeds.

 

[jbriggs444]

What, is Conservation of Angular Momentum not a thing anymore? Or has George Gamow been teaching me false information...

Conservation of momentum tells us that in order to spin twice as fast, some one or some thing would need to exert an external torque on the moon. So take that as a given. Someone or something has reached in and applied an external torque on the moon, thereby doubling its spin rate. That has little or nothing to do with the orbital velocity of a satellite going around the now faster-spinning moon.

 

[Nugatory]

What, is Conservation of Angular Momentum not a thing anymore? Or has George Gamow been teaching me false information...

Conservation of angular momentum is a real thing. It just doesn't have anything to do with the question in this thread.

 

[Comeback City]

Conservation of momentum tells us that in order to spin twice as fast, some one or some thing would need to exert an external torque on the moon. So take that as a given. Someone or something has reached in and applied an external torque on the moon, thereby doubling its spin rate. That has little or nothing to do with the orbital velocity of a satellite going around the now faster-spinning moon.

Okay I see now. I guess I misconceived what the OP was proposing. Thanks for clearing that up.

 

[CWatters]

I've never studied relativity but I think the fact that the Earth is rotating increases it's mass very slightly and hence increases gravity very slightly. The effect must be very very small.

 

[davenn]

but I think the fact that the Earth is rotating increases

it's actually slowing ... day's used to be somewhat shorter long ago

from wiki ...

Analysis of historical astronomical records shows a slowing trend of 2.3 milliseconds per century since the 8th century BCE

Change in rotational velocity[edit]
Tidal interactions[edit]
Over millions of years, the Earth's rotation slowed significantly by tidal acceleration through gravitational interactions with the Moon. In this process, angular momentum is slowly transferred to the Moon at a rate proportional to r − 6 {\displaystyle r^{-6}} [PLAIN]https://wikimedia.org/api/rest_v1/media/math/render/svg/5562d12471ec1ca2053ab569f0e5c1451f315d62, where r {\displaystyle r} https://wikimedia.org/api/rest_v1/media/math/render/svg/0d1ecb613aa2984f0576f70f86650b7c2a132538 is the orbital radius of the Moon. This process gradually increased the length of day to its current value and resulted in the moon's being tidally locked with the Earth.

This gradual rotational deceleration is empirically documented with estimates of day lengths obtained from observations of tidal rhythmites and stromatolites; a compilation of these measurements[41] found the length of day to increase steadily from about 21 hours at 600Myr ago[42] to the current 24 hour value. By counting the microscopic lamina that form at higher tides, tidal frequencies (and thus day lengths) can be estimated, much like counting tree rings, though these estimates can be increasingly unreliable at older ages.[43]

Dave

 

[Ion Aguirre]

Centrifugal force does not only affect the value of gravity on a planet, but the direction of its vector.
If a weight is suspended by a rope, we all assume it points in a vertical line, but that's not right. Let's assume the planet as a sphere ... If the line drawn by the rope were extended downwards, it would not cross through the center of the planet.
Hence ... that vertical line, is not a geometric vertical, but only an apparent one.
This divertion of the vertical line has, for example, very strong effects on atmospheric circulation and winds.

 

[cosmik debris]

If you add angular momentum to a planet does it not increase its energy and therefor modify its energy-momentum tensor, even if it is negligible for the situation in the OP?

 

[CWatters]

it's actually slowing ... day's used to be somewhat shorter long ago

from wiki ...

Dave

I know the Earth rate of rotation is slowing. I was referring to the fact that Relativity says an object gains mass when its moving. So I think a planet has more mass and gravitational pull when it's rotating than when the same planet is not rotating.

 

[davenn]

I was referring to the fact that Relativity says an object gains mass when its moving.

but we are hardly talking about relativistic speeds here, aye

 

[DrStupid]

So I think a planet has more mass and gravitational pull when it's rotating than when the same planet is not rotating.

The source of gravity in general relativity is not mass but the stress-energy-tensor. That makes the relation between spin and gravity a bit more complicate.

 

[anorlunda]

The source of gravity in general relativity is not mass but the stress-energy-tensor. That makes the relation between spin and gravity a bit more complicate.

Maybe I can help the OP to phrase the question better.

Does the

Edit: Does the invariant mass of an object change when it spins?

 

[DrStupid]

Does the invariant mass of an object change when it spins?

Yes, the invariant mass of a closed system changes when it's rotational energy changes. But I'm not shure that this is what the OP is asking for. The original question is about the orbit of a satellite and that doesn't depend on the invariant mass only but on the complete stress-energy-tensor and of course on the shape of the planet (which also changes with the spin).

 

[Ion Aguirre]

Adding speed or momentum does certainly increase energy. But how would that momentum be added ? In fact, there is a substraction, but not an addition.

Rotational speed of the Earth and Earth-Moon system in slowering down, due to energy being continuosly "stolen" by the Tide effect from gravitational forces. Due to this same effect, the distance Moon-Earth increases as well.
Big Earthquakes, other celestial bodies, etc ... can also change orbital parameters.

Nothing measurable to do with Relativity, but with classic dynamics.

 

[DrStupid]

Rotational speed of the Earth and Earth-Moon system in slowering down, due to energy being continuosly "stolen" by the Tide effect from gravitational forces.

That's another topic.

 

[Ion Aguirre]

That's another topic.

My apologizes