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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 and Fg=Gm1m2/r^2

    3. The attempt at a solution

    There was no answer, i know im supposed to calculate the the velocity and its mass, then but to get the percent of mass missing i think you divide the difference between the the calculated mass and given mass by the calculated mass or something like that. But i am still rather confused at what to do. Im not looking for someone to complete my homework, just to help me understand how to do this question.
  2. jcsd
  3. Dec 9, 2013 #2


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    2017 Award

    Staff: Mentor

    Start with the part you described - calculate the mass with the formulas. It will be larger than 1042 kg, and this number is (visible mass)+(invisible mass). Can you calculate the invisible mass?
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