universal law of gravitation(variables)

[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

[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.

[captainsmith1]

thanks, i understand now