# Homework Help: Find the distance from the sun to the planet known the orbital period problem

1. Sep 27, 2009

### subopolois

1. The problem statement, all variables and given/known data
the problem asks to find the distance from the sun to a planet. the only information the question gives the the orbital period of 27 years and asks for the answer in km.

2. Relevant equations
c= (2)(pi)(r)

3. The attempt at a solution
assuming the planet revolves around the sun, i was assuming this is a simple problem using the circumference of a circle and solver for r to get the radius which is also the distance to the sun. heres my attempt.
c=(2)(pi)(r)
27=(2)(pi)(r)
r= 27/(2)(pi)
r= 4.29

thats my solution, but is seems really close to the sun, 4.29 km. have i done something wrong? can someone help?

2. Sep 28, 2009

### Staff: Mentor

Take a look at units - your result is 4.29 years, not kilometers. So obviously there is something wrong.

This is either application of Kepler's third law (but then you need infomation about some other planet distance and period) or of Newton's law of universal gravitation combined with equations of motion (but then you need mass of the Sun). Not knowing in what context this question has been asked it is not possible to decide which approach will be correct (although they will both give the same result, as Kepler's laws can be derived from Newton's law).

3. Oct 3, 2009

### the pro

yup it is calculatet in years not in km