Gravitational and electric fields

1. Oct 20, 2009

confusedpilot

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..

2. Oct 20, 2009

Donaldos

Where did that expression come from?

Find an expression for the radial acceleration in terms of the orbital speed and R.

Then use Newton's second law.