On the surface of the Moon, the free-fall acceleration is roughly one-sixth that of the Earth. Their average densities are 0.667 apart.
Given the radius of the Moon is (Rm) = 0.250 RE
The radius of the earth (RE) = 6.4 x 10^6 m
Let freefall acceleration of the moon = gm
Let freefall acceleration of the earth = ge
Given gm = ge/6
We know that g = Gm/r^2 where G is the gravitational constant
Let mass of moon = m1 and mass of earth = m2
Gm1/Rm^2 = Gm2/6xRE^2
mass = densityxvolume
let density of moon = d1 and density of earth = d2
d1 x (4/3Rm^3)/Rm^2 = d2x (4/3RE^3)/6xRE^2
d1xRm = d2xRE/6
d1/d2 = RE/6X0.250RE
d1/d2 = 0.667
Hence the ratio of densities of moon and earth is 0.667
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