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Q. The period of revolution of an earths satellite close to surface of earth is $90 \min$. The time period of another satellite in an orbit at a distance of four times the radius of earth from its surface will be

AIIMSAIIMS 2009

Solution:

From Kepler's law
$T^{2} \propto R^{3}$
or $ T \propto R^{3 / 2}$
$\frac{T'}{T}=\left(\frac{R'}{R}\right)^{3 / 2}$
or $ \frac{T'}{T}=\left(\frac{4 R}{R}\right)^{3 / 2}$
$=(4)^{3 / 2}=\left(2^{2}\right)^{3 / 2}=2^{3}=8$
$\therefore T'=8 T=8 \times 90$
$=720 \min$