# Gravitational and electric fields

## Homework Statement

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

## Homework Equations

I'm thinking keplers third law?

## The Attempt at a Solution

C=2piR

P=2piR/ sqrt*GMm/r

rearranged to give P = 2pi sqrt*R^3/GM

Lost now..

P=2piR/ sqrt*GMm/r

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.