# Thread: Dark Matter, calculating missing mass

1. 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?

Equations needed i think are Fc=mv^2/r and Fg=Gm1m2/r^2

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. Originally Posted by cherioslover 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?

Equations needed i think are Fc=mv^2/r and Fg=Gm1m2/r^2

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.
You have a general mass, m, in the formula for Fc and two specific masses, m1 and m2, in the formula for Fg. I think you need to have a diagram of the object whose acceleration you are calculating so that you know which masses correspond. You are then going to have to think about exactly how you get a position for the mass which is exerting the force Fg that causes the acceleration of that object.

Also, how are Fc and Fg related?

Ideally, doing this would get you oriented. However, depending on what physics you know you may need more hints. Let us know whether or not I have given you enough of a hint to get you started.

Incidentally, as you have written the problem, the answer you are looking for is not a ratio but the actual mass. There is no division of masses needed.  3. Thanks for your help, i eventually figured it it out . And the ratio part was only to find the percent of mass missing, so what percent of the mass was unaccounted for as well as the actual value.  4. Originally Posted by cherioslover Thanks for your help, i eventually figured it it out . And the ratio part was only to find the percent of mass missing, so what percent of the mass was unaccounted for as well as the actual value.
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