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Formula for gravity/circular motion in terms of V? (i think )

  1. Nov 16, 2008 #1
    If F=mg, and Fc=Fg, Fc=mv2/r (thats v squared; my apologies, i have no idea how to use the superscript button), and Fg=Gm(1)m(2)/r2 (squared), where m(2) is the mass of, say, the earth, and m(1) is the mass of the object orbiting the earth. By substituting these equations together, we get


    and we can cancel out the r on the left, and the m (which is mass). but I have a problem; which mass is canceled out on the right side? is it the mass of the object or mass of the earth?

  2. jcsd
  3. Nov 16, 2008 #2

    Doc Al

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    Staff: Mentor

    Which do you think? Which mass is executing circular motion as it orbits the other? Which mass are you finding the centripetal force on?
  4. Nov 16, 2008 #3
    I think i am supposed to be finding it with respect to the earth, so I believe then the mass of the earth is what is left
  5. Nov 16, 2008 #4

    Doc Al

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    Staff: Mentor


    Force on orbiting object due to gravity: [tex]F = GM_{earth}M_{object}/R^2[/tex]

    Applying Newton's 2nd law to orbiting object: [tex]F = M_{object}V_{object}^2/R[/tex]


    [tex]M_{object}V_{object}^2/R = GM_{earth}M_{object}/R^2[/tex]

    [tex]V_{object}^2/R = GM_{earth}/R^2[/tex]
  6. Nov 16, 2008 #5
    Cancel out mass of object in revolution,thats the mass acted upon by the centripetal force.
  7. Nov 16, 2008 #6
    thanks a lot! that really helped.
    next time ill try to not leave my physics book at school over the weekend
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