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I Effect of rotational inertia on gravity or visa versa

  1. Mar 4, 2017 #1
    Hi,
    I wonder if a gravitationally bound system ...like a binary star system (or a spinning galaxy for that matter) has a similar inertia in its rotational axis like your classic spinning top? Is there resistance to a change in the axis?
    I appreciate the system is not solid and so not obviously analogous but is there even a small net torque on the rotational axis of gravitationally bound bodies when the orbiting bodies axis is askew as compared with the encompassing rotational axis/gravity vector?
    Isn't the body (of the orbital system) that is nearer the galaxy centre of mass (at any one time) then being pulled away from its orbital partner which then adjusts ...
    I'm thinking maybe there is a tendency to preserve the rotational axis direction ... since otherwise wouldn't older galaxies have the rotational plane of its solar systems more radially aligned (flattened) with respect to the galaxy rotation? (perhaps they do?)
    If there is a net torque wouldn't that torque on rotational axis be using up ..so to speak, the gravitational energy at that point as compared with gravity acting on an equivalent non rotating systems centre of mass ? ...Many thanks for any response assuming if I have made enough sense.
     
  2. jcsd
  3. Mar 4, 2017 #2

    mfb

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    Yes.
    Tidal gravity from the rest of the galaxy? Completely negligible.
    There is nothing that would do such an alignment, even for a disk that doesn't rotate, even if tidal gravity would be strong enough to have an effect (then you would just get wobbling).
     
  4. Mar 4, 2017 #3
    Thanks mfb - if the case has it that 2 galaxies of say diameter D approach each other - one on the horizontal, the other say 45 degrees to it, their centre of masses both lying on the same horizontal plane and their peripheries are say less than 10D apart which I imagine provides for a clear difference in felt gravity (from the other galaxy) between the leading and trailing edges.
    Isn't the leading edge (considering just the galaxy we're imagining is at 45 degrees ) going to be pulled down towards the horizontal plane relative to its galaxy's existing rotational axis? And as it rotates and different stars become that leading edge - and so that edge and all stars exposed to that edge are forced off their previous rotational plane ... does that cause, any change in the galaxy's rotational axis / is it possible to imagine a torque being applied to the rotational axis as a result of the leading edge perturbation? ...I'm slightly befuddled .. seems as if I'm trying to imagine a force that isn't gravity ..anyhow thanks for anything you might offer here. :)
     
  5. Mar 4, 2017 #4

    mfb

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    If they get too close, they deform and merge in complex ways. Galaxies are not solid disks, the individual parts can move in completely different directions. There are various simulations, e. g. this one or this one.
     
  6. Mar 4, 2017 #5
    nice link thank you - complex indeed. oh shucks my scenario didn't envisage such quick closing velocities relative to rotational speed ...
     
  7. Mar 4, 2017 #6

    mfb

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    Well, the rotational velocity is smaller than the escape velocity at that point. The galaxies cannot approach each other slower than the rotation velocities.
     
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