Calculate the value of acceleration due to gravity on a planet whose mass and radius are 2 times that of the earth. 

Consider, 
g_e = Acceleration due to gravity on the earth
g_p = Acceleration due to gravity on the given planet
M_e = Mass of the earth
R_e = Radius of the earth
M_p = Mass of the planet
R_p = Radius of the planet

Acceleration due to the gravity at the surface of the earth is:

$g_e=\frac{G{M}_e}{R_e^2}$

It is given that $M_p=2M_e,R_p=2R_e$

Acceleration due to the gravity at the surface of the planet: 

$g_p=\frac{G{M}_p}{R_p^2}=\frac{G\left(2M_e\right)}{(2R_e{)}^2}$ 

$g_p=\frac{G{M}_e}{2R_e^2}=\frac{g_e}{2}$

$g_p=\frac{g_e}{2}= 0.5 g_e$

$g_p= 0.5 g_e$

Therefore, the acceleration due to gravity on the planet will be 0.5 g that is half the value of the g on the earth.