Question

The radii of two planets are r and 2r respectively and there density p or p/2, what is the radio acceleration due to gravity at their surface ,at what height the acceleration due to become g/9.

Answer 1

Acc due to gravity at the surface of planet = g = (4/3) GRλp

Planet 1; R = R
p = P

so, 
g1 = (4/3) GRλP

planet 2 ; R = 2R
p = P/2
so, 
g2 = (4/3) G2RλP/2
or 
g2 = (4/3) GRλP

thus g1 = g2
ratio will be 1:1