# gravitation

Joined Dec 29, 2004
83
Hi , Please can you give me some ideas of how to do this problem:

Two planets are orbiting a star in a distant galaxy . The first has a semimajor axis of 150e6 km, an eccentrictity of e=.20 and a period of 1.0 Earth year.
What is the period of the second which has a semimajor axis of 25066km, an eccentricity of e2=.30

I get lost when I work on gravitation problem.

B

#### cookevillain

Joined Dec 1, 2005
4
Hi,

The second Kepler's law says that the square of the period is proportional to the cube of the major axis. So, if you are going to use this law, here is what you need to do:

1) find the lengths of major axes of both ellipses (any calculus book would have something about this)

2)Find the ratio r=(P1)^2/(A1)^3

3)Now you know that also r=(P2)^2/(A2)^3, and you also know A2 from 1).

Solve for P2.

C-villain

Originally posted by braddy@Dec 3 2005, 10:16 PM
Hi , Please can you give me some ideas of how to do this problem:

Two planets are orbiting a star in a distant galaxy . The first has a semimajor axis of 150e6 km, an eccentrictity of e=.20 and a period of 1.0 Earth year.
What is the period of the second which has a semimajor axis of 25066km, an eccentricity of e2=.30

I get lost when I work on gravitation problem.