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# The mass of earth is $6.00 \times 10^{24} \;kg$ and that of the moon is $7.40 \times 10 ^{22} \;kg \;$ and $G= 6.67 \times 10^{-11} Nm^2 /kg^2$. potential energy of the system is $-7.79 \times 10^{28} J$. The mean distance of between earth and moon is

$(a)\;3.80 \times 10^8 \;m \quad (b)\;3.37 \times 10^6 \;m \quad (c)\;2.60 \times 10^4\; m \quad (d)\;1.90 \times 10^2 \;m$

$V=\large\frac{-GMm}{r}$
$7.79 \times 10^{28}=\large\frac{6.67 \times 10^{-11} \times 7.4 \times 10^{22} \times 6 \times 10^{24}}{r}$
$r= 3.8 \times 10^8 m$