# Galactic mass proportional to r

1. Dec 8, 2003

### StephenPrivitera

How do I calculate galactic mass as a function of r? I know it is supposed to be proportional to r. I've seen,
M(r)=v2r/G
And also I've seen
M(r)=vrc2/G
Which if any is right?
And what do I use for v?
Thanks so much for your help!

2. Dec 8, 2003

### StephenPrivitera

The rotation curve is flat so I can find v by plugging in the radius and total mass of the galaxy. Doing that I get,
M(r)=Mr/R, where M is the total mass of the galaxy and R is the radius of the galaxy
I'm trying to do a tidal force calculation to see how far away from the center of a galaxy a star needs to be to be tidally disrupted by a nearby galaxy. I'm getting
r=(d3m/(2MR))1/2 where m is the total mass of the galaxy that the star lies in, R is the radius of that galaxy, M is the total mass of the nearby galaxy, and d is the distance between the two galactic centers. Can anyone verify this?