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

1. Apr 26, 2010

### seto6

[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. Apr 27, 2010

### seto6

Re: [urgent] finding mass enclosed my a stars radius from center.

3. Apr 27, 2010

### zachzach

Re: [urgent] finding mass enclosed my a stars radius from center.

Here's my take:

Why not try a ratio?

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

Ratios are always good in astronomy.

But v is not proportional to M/R, it is proportional to $$(\frac{M}{R})^{1/2}$$ since it was given in the problem that $$v = (\frac{GM(r)}{R})^{1/2}$$

Last edited: Apr 27, 2010