A 21.0-kg satellite has a circular orbit with a period of 2.37 h and a radius of 8.50×106 m around a planet of unknown mass. If the magnitude of the gravitational acceleration on the surface of the planet is 7.10 m/s2, what is the radius of the planet?
Use Kepler's third law (with period and orbit radius) to get the Mass of the planet.
The gravitation acceleration at the planet's surface is g' = GM/R^2.
Since you now know g' and M, you can solve for R. (G is the universal gravitational constant.)
great! thanks again for your help :)