# Planetary Motion? I think?

1. Nov 29, 2008

### omgitsmonica

1. The problem statement, all variables and given/known data
"Yoda has just discovered planetary system consisting of star Dagobahr and its two planets: Rool and Krau. Planet Rool has average orbital radius 1.6 times as big as planet Krau. If planet Rool orbits Dagobahr in 2.9 Earth years, how long is a year for the resident of Krau? Give the answer in Earth years.

2. Relevant equations
Trool^2/rRool^3 = TKrau^2/rKrau^3

3. The attempt at a solution

TRool =?
rRool = 1.6*rKrau
Tkrau=2.9
rkrau=?

I have no idea where to go from here...

2. Nov 29, 2008

### Janus

Staff Emeritus
Try re-arranging your equation so that the orbital radii are on the same side. What happens if you substitute your values into the equation?

3. Nov 29, 2008

### horatio89

Kepler's Third Law can be rewritten as R = kT2/3, where k is the constant of proportionality. When R increases by 1.6, by what factor does T increase?

Last edited: Nov 29, 2008
4. Dec 1, 2008

### omgitsmonica

Does T increase by 1.6, too?

5. Dec 1, 2008

### horatio89

That is incorrect. T would increase by 1.6 if T and R are directly proportional. However, according to Kepler's Law, R is proportional to T^2/3.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook