# Uniform circular motion with unknown mass

1. Jun 13, 2007

### Emil Zapotec

1. The problem statement, all variables and given/known data
A satellite is in a circular orbit around an unknown planet. The satellite has a speed of 1.7x10^4 m/s and the radius is 5.25x10^6 meters. A second satellite also has a circular orbit around the same planet and has an orbit radius of 8.6x10^6 meters. What is the orbital speed of the second satellite?

3. The attempt at a solution
I was wondering if I have to find the centripetal acceleration of the first satellite. If so I know the acceleration is 55.05 m/s^2. But I'm stuck on what to do with there because I don't even know if that acceleration is relevance and how you work it into mv^2/r

2. Jun 13, 2007

### Archduke

What force is causing the satellite to be in circular orbit? Do you know any equations for this force?

3. Jun 13, 2007

### Emil Zapotec

The only force would be the gravitational force of the unknown planet. But theres no information about the planet, only that a satellite rotates around it, so I'm assuming theres a formula that I need to derive where a mass needs to cancel?

4. Jun 13, 2007

### Archduke

As the gravitational force is the only force, how about equating it to the centripetal force?

5. Jun 13, 2007

### Emil Zapotec

Oh, so just find the centripetal force of the first one, find out what it is and equate it to the centripetal force of the satellite with the unknown velocity? I guess I made it a lot harder than I thought, thanks a lot.

6. Jun 14, 2007

### Archduke

Well, the centripetal force of the first satellite isn't the same as the centripetal force on the second satellite. But, once you've equated the centripetal force and gravitational forces together, can you re-write the expression so one side of the equation has only constants, and the other side has the variables. Now, as one side has only constants, you can equate the variables of the two satellites. I guess I'm saying use proportionality, but in a really round-about way!