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A rope in tension between Earth and Moon

  1. Feb 20, 2014 #1
    I attatch a rope of a constant density to the Moon and fasten it down to the surface. I set the other end in space right above Earth's atmosphere. The angular speed of the entire rope is brought up to the same angular speed as the Moon at time=0.


    What would happen to the rope? Would it stay pointing radially between moon and COM of earth? Why or why not?

    My gut says that there would be some force that would cause the rope to not point radially from some gravitational dragging effect but I would love some insight.


    no interaction from atmosphere (ideal vacuum)
    mass of the rope does not effect the rotation of either planet
    moon does not precess
    moon in circular orbit(would it matter?)
  2. jcsd
  3. Feb 20, 2014 #2


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    Science Advisor

    Tension in the rope would add a small attractive force between the Earth and the moon. There's some minimum tension, because one end of the rope falls toward the Earth, and one end of the rope falls toward the moon. Since the Earth pulls harder, the end of the rope on the moon will get pulled off the surface by the rope's tension, unless it were attached to the moon somehow.

    Are you interested in frame-dragging effects? Or just Newtonian gravity?
  4. Feb 20, 2014 #3
    Wouldn't the rope tear? I mean earths gravitational pull is stronger are we assuming the rope is strong enough to withstand the tension? Also is it attached to the surface or just touching?
  5. Feb 20, 2014 #4
    I said in the problem statement that I fastened 1 end of the rope to the moons surface.

    I was curious what would happen to the rope. I wanted to assume that the rope had mass to feel the effect of gravity, but that the tension in the rope would not signifacantly alter the orbital trajectory of the idealized moon(no precession and circular orbit).

    Would the rope always point toward the COM of Earth or is there some force that would deflect it?
  6. Feb 20, 2014 #5
    It is attached to the surface of the moon and the rope has infinite tensile strength.
  7. Feb 20, 2014 #6
    Not sure how strong it would be but depending on the mass of the rope centrifugal force would just push it back to the moon. If the gravity of the earth was less then the centrifugal then the rope would go back to the moon I believe, However if was more massive the gravitational force would overcome the centrifugal force correct?
  8. Feb 20, 2014 #7
    Yes, But... The gravitational force on the rope would always be greater than the centrifugal force on the rope. If this were not the case, the the moon would be pulled away from the Earth by its large orbital velocity.
  9. Feb 20, 2014 #8
    There is evidence that suggest that the moon is pulling away from out orbit though.
    http://lro.gsfc.nasa.gov/moonfacts.html CTRL-F search for drift or 1.5
    Last edited: Feb 20, 2014
  10. Feb 20, 2014 #9
    I am making the assumption of a circular lunar orbit of constant radius over time.
  11. Feb 20, 2014 #10
    The answer is very simple. Under the idealized conditions as described by the problem the rope would just stay there under incredible tension. You might find googling "space elevator" worthwhile.
  12. Feb 20, 2014 #11
    This is different then a space elevator because it is under gravity by both the moon and earth. A space elevator uses the centrifugal force to overcome gravity pressing down on it, meaning the problem he is stating can only be achieved using the two celestial bodies in question however a space elevator can be attained with only one body.
    Last edited: Feb 20, 2014
  13. Feb 20, 2014 #12
    Even so with the current velocity of the moon and the gravity of earth It would head back to the moon due to centrifugal force, but are you assuming that there are the right conditions for the rope to overcome the centrifugal force and not drift due to centrifugal force?
  14. Feb 20, 2014 #13
    Since I have only taken classical mechanics, that is the answer that I arrived at. Are you certain that there is no effect from General Relativity that would cause the rope to deflect?
  15. Feb 20, 2014 #14
    There is frame drag due to the rotation of the Earth. Too small to worry about.
  16. Feb 20, 2014 #15
    You're forgetting about Earth's gravity
  17. Feb 20, 2014 #16
    The total centrifugal force on the rope is waaaaay weaker than the force from Earth's gravitational field.
  18. Feb 20, 2014 #17
    Too small to worry about? But that is the whole point of my question. If there is an effect, however small, then the rope would not point radially.

    So I guess the answer to my question is...

    No, the rope would not point toward the COM of the Earth. Because there would be a small force(very small) from frame dragging, which would cause a non-radial force on the end of the rope. This force would cause a small deflection which would cause the rope to point somewhere other than the COM of Earth.

    Is that correct?
  19. Feb 20, 2014 #18
    We would have to observer the rope, earths current gravity the moon is escaping, however would the rope escape the same way the moon is or since it is closer would it stay? It might stay but if that was true wouldn't the moon be staying as well. since the speed and distance is proportional to the speed and distance at both ends of the rope, would the stronger gravity at the end not attached to the moon be enough to keep it in orbit or would the tip come towards the moon like the moon is, that depends on the length of the rope I guess we assume that the length is long enough for gravity to overcome the centrifugal force.
  20. Feb 20, 2014 #19
    Also there is the fact that there is a small push on the rope from the sun, the rope would act as a small solar sail.
  21. Feb 20, 2014 #20
    Yes, I think so.

    If that's the point of the question than you should've asked it at the relativity forum
  22. Feb 20, 2014 #21
    Thanks I'll do that.
  23. Feb 21, 2014 #22


    Staff: Mentor

    It would be deflected.

    There would be a very small force due to frame dragging, caused by the Earth's rotation, but this would be much too small to measure.

    However, you've ignored a much bigger force: since you've forced the rope to have the same angular velocity everywhere at time t = 0, different parts of the rope, at different altitudes, will have different *linear* velocities, so there will be stress in the tangential direction (i.e., perpendicular to the rope) between any two neighboring pieces of the rope at time t = 0. In Newtonian terms, this is called the Coriolis force. Since the lower end of the rope (the end closest to the Earth) is free to move, it will do so in response to the Coriolis force, deflecting the rope.
  24. Feb 21, 2014 #23
    Which way would the stress deflect the rope? And would it oscillate?
  25. Feb 21, 2014 #24
    Here is how I'm thinking about it...

    Average lunar distance LD = 238,900 miles
    Rope's center of mass = 119,450 miles from Earth
    Lagrange point L1 = 200,900 miles from the Earth

    84% of the rope's mass is on the Earth side of L1
    (good reason to fasten it down to the Moon's surface)

    Orbital period of L1 = the Moon's orbital period
    Rope between L1 and Earth will try to progressively decrease orbial period with approach to Earth, which it can't do, so it will curve "forward" of the line between the Earth and Moon.

    Rope between L1 and Moon will try to increase orbital period with approach to Moon, which it can't do, so it would tend to make the rope fall "behind" the line between the Earth and Moon; but the fixed end fastened to the Moon surface is holding that to form a tight curve.

    Once the rope finds its static shape, I think with respect to your diagram, the rope will look like a "sword"... pointing left and down, the long part on the Earth side of L1 will be like the blade and curve gradually "up" as it approaches the Earth, and the small section between L1 and the Moon will curve "down" in a half loop like the knuckle guard on the hilt.
  26. Feb 21, 2014 #25

    If I understand your odd description correctly, is it like this picture?

    I'm having a hard time picturing it.
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