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Homework Help: Dark Matter, calculating missing mass

  1. Dec 8, 2013 #1
    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. jcsd
  3. Dec 8, 2013 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    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?

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