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B Can a force generate rotation with moment arm=0 ?

  1. Oct 25, 2016 #1
    I wait for your thoughts on this topic... and maybe then I will give you a small surprise.

    edit:

    Ok, there was a misconception on my side that I want to clarify changing the original title by this one:

    If there are two planets, rotating one around the other, and only gravity is acting. Is it possible that between them the net force is not acting between their centers?
     
    Last edited: Oct 25, 2016
  2. jcsd
  3. Oct 25, 2016 #2
    Okay, I'll bite with the expected answer: no, since you need a torque to change the angular momentum and you need a moment arm to have a torque.
     
  4. Oct 25, 2016 #3
    Ok, and what if I say to you that the two systems are Earth-Moon?
     
  5. Oct 25, 2016 #4

    Drakkith

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    I'd say you need to clarify what you're talking about.
     
  6. Oct 25, 2016 #5
    Done!
     
  7. Oct 25, 2016 #6
    Im not sure i understand what you mean, but if earth and moon had arm=0 i dont think we would be here writing on the forum right now
     
  8. Oct 25, 2016 #7

    Drakkith

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    Staff: Mentor

    As far as I know, a planet or moon (or other body) whose mass is not distributed evenly will not have the net force acting through the center of its volume. This can (and does) result in a torque.
     
  9. Oct 25, 2016 #8
    Well I was reading about tidal friction, a topic I found really interesting.

    https://en.wikipedia.org/wiki/Treatise_on_Natural_Philosophy#cite_note-4

    It is in volume 1, article 276, a nice read folks!
     
  10. Oct 26, 2016 #9

    CWatters

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