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Homework Help: Finding mass enclosed my a stars radius from center.

  1. Apr 26, 2010 #1
    [urgent] finding mass enclosed my a stars radius from center.

    1. The problem statement, all variables and given/known data
    the rotational speed of an orbiting object is given by v= (Gm/R)^1/2where R is distance at which the object is rotating from the center of the mass distribution and M is the amount of mass within the radius R. for the distribution in a disk, the mass enclosed increases with R^2. the sun rotates in the ids of the Galaxy at distance of 8Kpc and a speed of 220km/s find the rotational speed of a star in the disk that is located at a distance of 12Kpc from the center of Galaxy.

    i have no clue how to do this question. it was on my quiz but didn't know how to do. i really need help.


    2. Relevant equations

    the equation is given in the question.

    3. The attempt at a solution
    i have no clue how to start this.
    i tried using v is proportional to M/R but it does not work
     
  2. jcsd
  3. Apr 27, 2010 #2
    Re: [urgent] finding mass enclosed my a stars radius from center.

    Please help, i would greatly appreciated, i have a exam tomorrow, please.
     
  4. Apr 27, 2010 #3
    Re: [urgent] finding mass enclosed my a stars radius from center.

    Here's my take:

    Why not try a ratio?

    [tex] \frac{v}{v_{sun}} = (\frac{GM(r)}{R})^{1/2}(\frac{R_{sun}}{GM(r)})^{1/2}
    \rightarrow v = v_{sun}(\frac{R_{sun}}{R}} )^{1/2} [/tex]

    Ratios are always good in astronomy.

    But v is not proportional to M/R, it is proportional to [tex] (\frac{M}{R})^{1/2} [/tex] since it was given in the problem that [tex]
    v = (\frac{GM(r)}{R})^{1/2} [/tex]
     
    Last edited: Apr 27, 2010
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