How can tidal friction on Earth affect the Moon?

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Discussion Overview

The discussion centers on the effects of tidal friction on Earth's rotation and its influence on the Moon's orbit. Participants explore the mechanisms by which changes in angular momentum due to tidal forces are communicated between the Earth and the Moon, as well as the implications of hypothetical scenarios affecting Earth's rotation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asserts that the slowing of Earth's rotation due to tidal friction must result in an increase of angular momentum for the Moon, leading to its increased distance from Earth and decreased linear velocity.
  • Another participant mentions that ocean tides exert a gravitational force on the Moon, suggesting a connection between tidal forces and lunar motion.
  • A participant explains that the gravitational attraction between non-aligned tidal bulges and the Moon increases its angular momentum, while also noting that if the Moon orbited faster than Earth's rotation, the opposite effect would occur.
  • One participant introduces the concept of torque exerted on Earth's equatorial bulge by the Moon and the Sun, suggesting that this torque facilitates the exchange of angular momentum between the two bodies.
  • Another participant raises the idea that the imperfect exchange of momentum due to friction forces in the precession of Earth's spin axis may have implications for the study of tidal interactions.

Areas of Agreement / Disagreement

Participants present multiple competing views on the mechanisms of tidal interactions and their effects on the Moon's orbit. The discussion remains unresolved regarding the completeness of explanations and the implications of hypothetical scenarios.

Contextual Notes

Some participants express uncertainty about the details of how tidal forces communicate changes in angular momentum, and the discussion includes references to complex interactions that may not be fully understood or agreed upon.

smithpa9
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I know the following statement to be true, being proven both mathematically, and verified by astronomical observations. What I need help understanding is HOW. . .

"The law of Conservation of Angular Momentum requires that the slowing down of the rotation of the Earth around its axis caused by tidal friction must result in an equal increase of angular momentum of the Moon in its orbital motion around the Earth. . . [this] must result in the increase of its distance from the Earth and the decrease of its linear velocity." George Gammow, Gravity.

Here's the question: How is the impact of tidal friction on the Earth COMMUNICATED to the Moon? By what force is the change in angular momentum of the Earth communiated to the Moon, thus affecting it's angular momentum? Gravity? If so, how?

If by some supernatural occurrence, the Earth suddenly stopped spinning entirely, can I assume that the Moon would immediately move rapidly farther away in its orbit? If so, why? how? what would "push" it away? Would the gravitational attraction to the Earth somehow lessen ?

Thanks for any explanations in as laymen of terms as possible to a former student of first year physics only.
 
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The ocean tides also exert a gravitational force on the moon.

In the supernatural case, moon would not be affected, although the .. supernature.. would start spinning. It's basically Newton's third law.
 
Friction between the rotating Earth causes the Earth to drag the tidal bulges along with it. As a result, they do not line up exactly with the moon but lead it a little. It is the gravitational attraction between these non-aligned tidal bulges and the Moon that pulls forward on the moon, increasing its angular momentum.

If the Moon orbited the Earth faster than or in the opposite direction of the Earth's rotation the tidal bulges would lag behind the moon and you would get the opposite effect, with the moon losing angular momentum and getting closer to the Earth.
 
Last edited:
Janus said:
If the Moon orbited the Earth faster than or in the opposite direction of the Earth's rotation the tidal bulges would lag behind the moon and you would get the opposite effect, with the moon losing angular momentum and getting closer to the Earth.
Like Phobos, Triton, and the theorized lost moons of Venus.
 
Excellent. All good answers. Thank you.
 
I appreciate the answers but I guess there is more to it. The Earth spin axis follows a precession cone, to be completed every 26000 years. Newton has explained why, it's the torque exerted on the equatorial bulge by the gravitational force of the moon and a little of the sun.

Torque is able to exchange angular momentum (vector) between bodies and if you realize that every 13000 years the vector direction is at opposing ends, then it should be clear that enormous amounts of momentum (vector direction) have been exchanged. I could imagine that friction forces in this process also make that exchange imperfect. I wonder if these aspects have been studied.
 

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