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Homework Help: Mass of the Milky Way through the speed of a planet

  1. Sep 26, 2012 #1
    1. The problem statement, all variables and given/known data

    Is it possible to calculate the mass of the Milky Way knowing only the speed of a planet in a circular orbit? I fail to see how, but if yes, then how?

    Thanks in advance

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 26, 2012 #2
    The planet would have to be orbiting the Milky Way in the same way that the Sun and all the other stars in the MW are orbiting.

    The relationship between a planet and the star its orbiting has no connection to the mass of the Milky Way.
  4. Sep 26, 2012 #3
    I did type planet, indeed. Let it be a star then for clarity's sake. Think you get the point.
  5. Sep 26, 2012 #4
    well there's only one force acting on a something out in space (most of the time, let's just assume this is the case)

    so if Fnet = Fg

    can you take it from there?
  6. Sep 26, 2012 #5
    Well, in a circular orbit,

    You are given only the speed v. The problem is how to get rid of the radius (r). Dunno how, and if possible.
  7. Sep 26, 2012 #6
    What's the exact problem and what values are you given?
  8. Sep 27, 2012 #7
    Find the mass of the Milky Way, when an orbiting star's speed is 220 km/s.
  9. Sep 30, 2012 #8
    Im guessing there's no way to solve it then? Because a friend of mine asked me for help with this, and I immediately asked if she had forgot to copy down the radius of the orbit or the period of the orbit. She said it was not given, so I got kind of confused. I still cant see a way to solve it.
  10. Sep 30, 2012 #9


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    Exactly. If you don't have the radius, then you can't get the speed. You would need to find the radius some other way, or introduce other assumptions.
  11. Sep 30, 2012 #10
    What kind of assumptions could be introduced here?
  12. Sep 30, 2012 #11


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    for example, you could assume that the star fits the typical trend of other stars in orbit around the milky way, then you could work out the radius of the star. But from what you've said in previous posts, I am guessing you are not allowed any other information.
  13. Oct 1, 2012 #12
    Could you elaborate on this please? Seems interesting, but dont think I follow. What kind of trend and how do you work out the radius from that?

    Thanks in advance
  14. Oct 1, 2012 #13


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    I just mean a trend of orbital radius and orbital speed. So for example, if a star has a certain orbital speed, then the trend would suggest a typical orbital radius for that star.

    But your original problem was to find the mass of the milky way given only the orbital speed of the star, so this method would not be any help.
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