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Q. Two satellites $A$ and $B$ of masses $200 kg$ and $400 kg$ are revolving round the earth at height of $600 km$ and $1600 km$ respectively. If $T _{ A }$ and $T _{ B }$ are the time periods of $A$ and $B$ respectively then the value of $T _{ B }- T _{ A }$
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[Given : radius of earth $=6400 km$, mass of earth $\left.=6 \times 10^{24} kg \right]$

JEE MainJEE Main 2021Gravitation

Solution:

$T =2 \pi \sqrt{\frac{ r ^{3}}{ GM }}$
$T _{ A }=2 \pi \sqrt{\frac{(6400+600) \times 10^{3}}{ GM }}$
$T _{ A }=2 \pi \times 10^{9} \sqrt{\frac{7^{3}}{ GM }}$
$T _{ B }=2 \pi \times 10^{9} \sqrt{\frac{8^{3}}{ GM }}$
$T _{ B }- T _{ A }=\frac{2 \pi 10^{9}}{\sqrt{ GM }}[8 \sqrt{8}-7 \sqrt{7}]$
$=314 \times 4.107$
$=1289.64$
$=1.289 \times 10^{3} s$