# Homework Help: Law of periods

1. Dec 3, 2006

### lizzyb

1. The problem statement, all variables and given/known data

Two planets A and B, where B has twice the mass of A, orbit the Sun in elliptical orbits. The semi-major axis of the elliptical orbit of planet B is two times larger than the semi-major axis of the elliptical orbit of planet A.

What is the ratio of the orbital period of planet B to that of planet A?

2. Relevant equations

$$T^2 = (\frac{4 \pi^2}{G M}) r^3$$

3. The attempt at a solution

$$M_B = 2 M_A$$
$$a_B = 2 a_A$$
$$\frac{T_B}{T_A} = \frac{\sqrt{\frac{4 \pi^2}{G 2 M_A} 8 a_A^3}}{\sqrt{\frac{4 \pi^2}{G M_A} a_A^3}} = \sqrt{\frac{8}{2}} = \sqrt{ 4 } = 2$$
but that was wrong. ????

2. Dec 3, 2006

### Andrew Mason

Planetary Mass is not a factor. Kepler's Third law states that $T^2/a^3$ is the same for all planets. The M in your equation is the mass of the sun.

AM

3. Dec 3, 2006

thank you.