Mysterious Momentum

[AbnormallyHonest]

So they say Phobos, the inner moon of Mars, is on a collision course with Mars. Ok, I don’t disagree with that. I also do not doubt that this deteriorating orbit has something to do with the orbital period being lass than the rotation of the planet.

My question is, Phobos is tidally locked to Mars, and it’s rotation vs revolution are synchronous, where does the angular momentum come from?

With a deteriorating orbit, as the moon falls toward the surface, the orbital period will become shorter. As that happens, wouldn’t the frequency of rotation need to increase in order to remain synchronous with the orbital period? Where does the energy for that increase come from? Is it possible that the energy to maintain that synchronicity is the cause of the deterioration? Could Phobos, at some point lose its tidal lock and begin a rotation that is less than its revolution? If that were to happen could this halt the deterioration?

If you believe that the moon creates a bulge that... blah, blah, blah.... then you doubt Issac Newton’s Gravitational Constant or Carl Guass who also had a law that states,

“The gravitation flux between any surface is proportional to the enclosed mass.”

To calculate a bulge that transfers energy from any vector other than the center of mass goes against both these well accepted universal laws of physics.

Edited December 10, 2020 by AbnormallyHonest
Forgot the quote.

[Bufofrog]

38 minutes ago, AbnormallyHonest said:

My question is, Phobos is tidally locked to Mars, and it’s rotation vs revolution are synchronous, where does the angular momentum come from?

Phobos is not in a synchronous orbit with mars.  It orbits faster than mars rotates, therefore tidal forces decelerate phobos.  The angular momentum is transferred to mars causing it to rotate slightly faster.

Edited December 10, 2020 by Bufofrog

[swansont]

56 minutes ago, AbnormallyHonest said:

If you believe that the moon creates a bulge that... blah, blah, blah.... then you doubt Issac Newton’s Gravitational Constant or Carl Guass who also had a law that states,

“The gravitation flux between any surface is proportional to the enclosed mass.”

To calculate a bulge that transfers energy from any vector other than the center of mass goes against both these well accepted universal laws of physics.

Gauss’s law assumes spherical symmetry. If you don’t have that symmetry (from a tidal bulge, for example) then it gets more complicated. You can have a torque, which allows for changes in angular momentum. The force is no longer completely radial.

[AbnormallyHonest]

11 hours ago, Bufofrog said:

Phobos is not in a synchronous orbit with mars.  It orbits faster than mars rotates, therefore tidal forces decelerate phobos.  The angular momentum is transferred to mars causing it to rotate slightly faster.

I’m not speaking on the synchronicity of Phobos’ rotation to Phobos’ revolution... not Mars. So you’r saying angular momentum is transferred to Mars? So on Mars the Sols are becoming shorter? I’ve never heard that before. So that would mean Phobos’ rotation is slowing down while it’s orbital period is accelerating, how does it maintain a tidal lock?

Edited December 11, 2020 by AbnormallyHonest
Typing errors.

[swansont]

5 minutes ago, AbnormallyHonest said:

I’m not speaking on the synchronicity of Phobos’ rotation to Phobos’ revolution... not Mars. So you’r saying angular momentum is transferred to Mars? So on Mars the Sols are becoming shorter? I’ve never heard that before. So that would mean Phobos’ rotation is slowing down while it’s orbital period is accelerating, how does it maintain a tidal lock?

I would imagine it’s similar to how the moon maintains tidal lock with the earth, even as it slows the earth’s rotation

[AbnormallyHonest]

10 hours ago, swansont said:

Gauss’s law assumes spherical symmetry. If you don’t have that symmetry (from a tidal bulge, for example) then it gets more complicated. You can have a torque, which allows for changes in angular momentum. The force is no longer completely radial.

Yes this is true. Tidal forces could cause a translation in the center of gravity. Besides the fact that these forces are acting laterally in relation to Phobos, and dismissing the fact that tidal effect takes place on both the facing and opposing side of mass, or the fact that the forces acting on earth are much more powerful and are acting on a surface that is 80% liquid, which is far more viscous than the nearly dead core of the entire planet Mars, you still have the the point that the center of mass is still the center of mass. That’s one point each... please explain how two (only two) points exert “torque” on one another?

[swansont]

4 minutes ago, AbnormallyHonest said:

Yes this is true. Tidal forces could cause a translation in the center of gravity. Besides the fact that these forces are acting laterally in relation to Phobos, and dismissing the fact that tidal effect takes place on both the facing and opposing side of mass, or the fact that the forces acting on earth are much more powerful and are acting on a surface that is 80% liquid, which is far more viscous than the nearly dead core of the entire planet Mars, you still have the the point that the center of mass is still the center of mass. That’s one point each... please explain how two (only two) points exert “torque” on one another?

When your object is not spherical (symmetrical), Gauss’s law doesn’t work. The objects aren’t points, so mass away from the center exerts a force.  r x F is torque

[AbnormallyHonest]

Guass’ Law assumes a sphere, but Newton’s did not. All mass, no matter how irregular, has a center of gravity. That is one point.

[AbnormallyHonest]

6 hours ago, AbnormallyHonest said:

Guass’ Law assumes a sphere, but Newton’s did not. All mass, no matter how irregular, has a center of gravity. That is one point.

The reason for this “torque” that was mentioned is due to the discrepancy between the center of gravity and the center of mass. For most of our everyday purposes we can consider these two thing synonymous, because we live and exist in a uniform gravitational field. Celestial bodies experience gravity on a much more dynamic scale, and therefore this can shift the center of gravity away from the center of mass. This discrepancy is what causes tidal forces.

[swansont]

10 hours ago, AbnormallyHonest said:

Guass’ Law assumes a sphere, but Newton’s did not. All mass, no matter how irregular, has a center of gravity. That is one point.

But Newton's law does not specify the center of mass. It tells us that every bit of mass attracts every other bit of mass.

We use center of mass because Gauss's law is usually a very good approximation. But it is still an approximation if you don't have the symmetry, and the torque comes from the deviations from that symmetry.

3 hours ago, AbnormallyHonest said:

The reason for this “torque” that was mentioned is due to the discrepancy between the center of gravity and the center of mass. For most of our everyday purposes we can consider these two thing synonymous, because we live and exist in a uniform gravitational field. Celestial bodies experience gravity on a much more dynamic scale, and therefore this can shift the center of gravity away from the center of mass. This discrepancy is what causes tidal forces.

And the tidal forces cause an asymmetry in the shape of the earth (or Mars, as the case may be). The mass that is no longer in a spherically symmetric distribution is at the surface - the tidal bulge - and that exerts a torque, because the force is not along the line between the centers of mass.

[J.C.MacSwell]

Here's how Swansont explained it to me some years ago. I seem to have lost the photo where he next picked up two spherical cows....