# Someone answer an orbital velocity question?

sonhouse
Science 05 Mar '09 06:07
1. sonhouse
Fast and Curious
05 Mar '09 06:07
I just read a short piece in New Scientist about a binary black hole system 5 billion LY away that seems to be mutually orbiting each other and they give the numbers as 0.3 LY apart and taking 100 years to orbit one another. Given they are about the same mass, they would be mutually orbiting one another, can you calculate the orbital velocity as if one were stationary and the other moving sort of like the Earth orbiting the sun? If so, I get a mutual orbital velocity of about 5600 Km/sec. Does that # hold up? 0.3 LY =~ 2.8 E12 Km radius, * 6.28 (2 PI) =~ 1.75 E13 Km circumferance / 100 years (3.1E9 seconds)=~5600 Km/sec,
if you consider one not moving. Is that a valid assumption? If not, how do you really calculate the mutual orbital velocity of two equal orbiting masses?
2. 05 Mar '09 06:45
Originally posted by sonhouse
I just read a short piece in New Scientist about a binary black hole system 5 billion LY away that seems to be mutually orbiting each other and they give the numbers as 0.3 LY apart and taking 100 years to orbit one another. Given they are about the same mass, they would be mutually orbiting one another, can you calculate the orbital velocity as if one were ...[text shortened]... n? If not, how do you really calculate the mutual orbital velocity of two equal orbiting masses?
Without doing any calcuation, the assumption that the two eqaually massed black holes, of which one is stationary and fixed in space, must be false.

However, in order to keep the fomula simple I can agree to the assumptions, but then we have to make an error estimate accordingly.
3. 05 Mar '09 18:10
If the black holes are equally massive, they will orbit around their center of mass, if you assume the orbit is circular you can get the speed by using half the distance between the black holes as the radius of the circle.
4. DeepThought