#### jimbo71

1. Homework Statement
see problem 9 attachment

2. Homework Equations
see problem 9 attempt attachment

3. 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.

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#### 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

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