Orbital problem giving answer much smaller than choices

[jimbo71]

Homework Statement

see problem 9 attachment

Homework Equations

see problem 9 attempt attachment

The Attempt at a Solution

I come up with a solution that is much smaller than any of the choices. I do not know if I am to assume the orbital is circular. I do not know if there is another error besides that in my work.

 

[horatio89]

Hello,

You need not assume that the orbit is circular. Use one of Kepler's Laws (which applies for both circular and elliptical orbits).

 

[jimbo71]

ok if I use kepler's third law would I start by using moon II orbital period and radius to solve for the mass of the planet?

 

[horatio89]

You don't need to solve for the planet's mass. Remember that R^3 is proportional to T^2, and that must hold for all objects orbiting a body. Hence R^3/T^2 = constant.

 

[jimbo71]

ok so I can find that ratio from moon II data and then multiply that ratio by T^2 of moon III?

 

[horatio89]

yup, that is right

 

[jimbo71]

thank you for your help