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Homework Help: Sun and Earth Problem

  1. Nov 28, 2007 #1
    More sample exam problems...

    2) The mass of our sun is ~ 2 x 10^30 kg. The mass of a planet is 6 x 10^24 kg. If the
    distance between the center-of-mass of each is 100 x 10^11 m, what is the distance
    in meters from the center-of-mass of the sun to the center-of-mass of the Sun-
    Earth system?

    What should I use to go about this problem? Thanks!
  2. jcsd
  3. Nov 28, 2007 #2
    Well, you could start by writing down some relevant formulas, such as:

    [tex]F_{g} = \frac{Gm_{1}m_{2}}{r^{2}}[/tex]

    [tex]\frac{T_{A}^{2}}{r_{A}^{3}} = \frac{T_{B}^{2}}{r_{B}^{3}}[/tex]
  4. Nov 28, 2007 #3
    What is the second equation? I'm not familiar with that one.
  5. Nov 28, 2007 #4
    That is the third law of Johannes Kepler. ;-)
    r: sun-planet distance
    T: period
  6. Nov 28, 2007 #5
    That one would help. So how would you relate them together? Energy Principle?
  7. Nov 28, 2007 #6


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    Homework Helper

    The second equation is what happens as a result of some fancy manipulation of centripetal force and gravitational force of attraction.
  8. Nov 28, 2007 #7
    ooh okay. I went to a help center and we had the centripetal force and gravitational force set equal to each other but the grad TA couldn't figure out how to get rid of velocity. So that second equation is really all i need right?
  9. Nov 28, 2007 #8


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    Use the centripetal force as [itex]m\omega^2r[/itex] where [itex]\omega=\frac{2\pi}{T}[/itex] to not have v and have the period of revolution
  10. Nov 28, 2007 #9
    will r be the distance from planet to center of mass?
  11. Dec 4, 2007 #10
  12. Dec 4, 2007 #11

    D H

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    Staff Emeritus
    Science Advisor

    I realize this is too late, but the help here has been quite bad. There is no need to invoke any of Kepler's laws to solve this problem. This is a simple problem. Quickclick, what is the equation of the center of mass for a group of particles?
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