1. The problem statement, all variables and given/known data By considering the centripetal force which acts on a planet in a circular orbit, show that T^2 is proportional to R~^3 where T is the time taken for one orbit around the sun and R is the radius of the orbit 2. Relevant equations I'm thinking keplers third law? 3. The attempt at a solution C=2piR P=2piR/ sqrt*GMm/r rearranged to give P = 2pi sqrt*R^3/GM Lost now..