Beginner Question

1. Oct 15, 2005

PhysicBeginner

Hi, everyone I'm new to physics and I got a problem that I can't solve.
It says
"What would the g constant be on a planet that has the same density as earth but has twice the radius?"

Can someone help me on this. Also can someone tell me if I'm suppose to use the equation: Fg=Gm1m2/d(square)?

2. Oct 15, 2005

confusedbyphysics

I'm new to physics too (so don't take my word for this, lol), but my guess is you set that equation above equal to ma, or in this case mg...

so mg = G m1 m2/ r^2, then the masses of the object on the planet cancel out so g = G m (planet)/ (2r)^2

since the radius is twice that of earth its just 2r..then plug in the values of G and r, and I'm not sure about mass of planet. if the densities are the same, are the masses are the same? if they are, then just plug in the mass of earth...and calculate g

hope this helps a bit until someone smarter comes along, lol

3. Oct 15, 2005

PhysicBeginner

Thanks for the tips, now i think I'm getting some of it.

4. Oct 15, 2005

whozum

If its looking for a numerical answer, I'd find the average density of Earth, find the radius, double the radius, then find the amount of mass that fits in that radius, and put all our new data for the imaginary earth into the equation you cited at the end.