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

Question

Near the surface of earth the acceleration due to gravity is 9.8 m/s^2. planet X has the same mass as the earth and twice the radius.

Answer 1

For a planet x of mass m and radius r, the acceleration of gravity is

g_x = Gm/r²

If R is the radius of the Earth, r = 2R, and m is the same as the mass of the earth, then

g_x = Gm / (2R)² = (1/4) (Gm/R²)

Note that Gm/R² is just the acceleration of gravity g on the surface of the Earth, so

g_x = g/4 = (9.8) / 4 m/s² = ? m/s²