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the radii of two planets are R and 2R respectively. and their densities are A and A/2 respectively. what is the ratio of acceleration due to gravity at their surfaces?

 volume of the planet v = (4/3 ) *3.14* r3
ie, The second planet is  8 times the volume of first planet 

density, p= m/v 

ie, The densities of the planets will be :
P1 = m/v    
 P2 = m/8v 
Considering the relation between given densities of planets 
P2 = P1/2, 

ie,  P1 / 2 =  m/8v 

Eliminate the half by multiplying out the opposite side of the second equation: 

Equating

m/v = 2m/8v = ( 1/4) * (m/v) 
That means  the second planet   has four times greater gravitational power as the first.


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