Satellite orbit homework problem?

Question

A satellite is in a circular orbit around an unknown planet. The satellite has a speed of 1.67 x 104 m/s, and the radius of the orbit is 4.39 x 106 m. A second satellite also has a circular orbit around this same planet. The orbit of this second satellite has a radius of 7.98 x 106 m. What is the orbital speed of the second satellite?

My work:
I had some crazy idea that proportions would work

V1/R1= V2/R2 which gave me 30356.7 which was incorrect

I really have no clue on how to approach this problem and any help given would be greatly appreciated. Thanks

Answer 1

see:
http://en.wikipedia.org/wiki/Kepler%27s_law#Kepler.27s_third_law

Answer 2

I don`t remember well the formula, but you can look for the right expressions once you get the idea:
In this ideal case, if a satellite is in a circular orbit, is because it`s in equilibrium its centripetal force with the planet`s gravitational force. So
(velocity satellite)^2/(orbit radius)*(mass satellite) equal to (gravity force first satellite).
from this, you should notice that this lead to a relation that doesn`t depends on the satellite mass. From this you can express the product (mass of planet*gravitational constant) as a function of the satellite speed and orbit radius.
Then, you could write the same for the second satellite (you`re not taking in account interactions between satellites). So
(velocity second satellite)^2/(second orbit radius)*(mass second satellite) equal to (gravity force second satellite).
Againt, this equation lead to a relation that doesn`t care about the second satellite mass. Now you can replace the product (mass planet*gravitational constant) found in the first satellite equation, and get the velocity of the second satellite.

Answer 3

I haven't worked orbital mechanics in about a hundred years, but I think your outer satellite is running at 2,202.9426 m/s.