# Dark Matter, calculating missing mass

1. Dec 8, 2013

### cherioslover

1. The problem statement, all variables and given/known data
The andromeda galaxy has a velocity of 250 km/s as it rotates. The observed mass is 1 x 10^42 kg. If the galaxy is roughly a circle with a diameter of 66,00 light years how much mass is not accounted for visibly?

2. Relevant equations

Fc=mv^2/R Fg=Gm1m2/R^2

3. The attempt at a solution

I know you're supposed to calculate the mass of the galaxy as well as a calculated speed, then divide the differences in masses by gravitational mass to find the percentage of mass missing, but honestly i cant seem to get it and there was no answer given :(

someone pleas be my hero :')

2. Dec 8, 2013

### Andrew Mason

Can you give us the exact wording of the problem?

If the question is stating that the mass on the outer edge of the galaxy is moving at a tangential speed of 250 km/sec, then you can work out the angular speed of the outer mass.

Does the problem state that you should assume that the galaxy is a disc of uniform density?

AM