Calculating Surface Gravity on Planet X?

[mandi182]

Homework Statement

Consider a hypothetical extra*solar world, planet X, that has 2.2 times the radius of the Earth and 2.5 times its density.

What is the acceleration due to gravity on the surface of planet X?

Homework Equations

g=GM/r^2

The Attempt at a Solution

I am not sure what to do with density. I know that mass/volume = density, but how do I solve the equation without mass? I know how to solve the equation when I know mass but if I only know density, how do I solve it?

 

[phyzguy]

If you know the radius, you can calculate the volume. If you know the volume and the density, you can calculate the mass...

 

[mandi182]

How do I use radius to calculate volume? What is the equation to do that?

Thanks

 

[phyzguy]

http://en.wikipedia.org/wiki/Volume_of_a_sphere#Volume_of_a_sphere

 

[mandi182]

Thanks, I wasn't able to get it right but I appreciate your help!