Orbital radius vs period question

[cheeselover87]

Homework Statement

a satellite has period T and orbital radius R. If you wish to increase the period to 8T what must the new orbital radius be? (answer is 4R, I just don't know how to get it)

Homework Equations

The Attempt at a Solution

 

[rock.freak667]

First think of the relationship between orbital radius and the period.

(Centripetal force = gravitational force of attraction)

 

[esc1729]

You may consider to use http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion#Third_law".

Erich