A spherical planet has radius R R\,R and uniform density ρ\rhoρ. The gravitational field strength at its surface is ggg. What is the gravitational field strength at the surface of a second spherical planet of radius 3R 3R\,3R and uniform density 4ρ4\rho4ρ?
3g3g3g
8g8g8g
12g12g12g
36g36g36g