# Universal law of gravitation(variables)

1. Nov 2, 2009

### captainsmith1

1. The problem statement, all variables and given/known data
A planet has twice the mass of earth, but both planets share the same acceleration of gravity. The radius of the new planet in terms of the radius R of earth is....?

2. Relevant equations
F=G(m1m2/r2), G being the universal gravitation constant

3. The attempt at a solution
i think you have to manipulate the equation in some way but i cant figure out what to do first.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 2, 2009

### ApexOfDE

From N's law, you have:

$$G\frac{Mm}{r^2} = ma_g$$

Because Earth and planet have same acceleration of gravity => a_g(E) = a_g(P) => relationship between masses and radii => radius of planet.

3. Nov 3, 2009

### captainsmith1

thanks, i understand now