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Every planet revolves around the sun in an elliptical orbit :
$A.$ The force acting on a planet is inversely proportional to square of distance from sun.
$B.$ Force acting on planet is inversely proportional to product of the masses of the planet and the sun
$C.$ The centripetal force acting on the planet is directed away from the sun.
$D.$ The square of time period of revolution of planet around sun is directly proportional to cube of semi-major axis of elliptical orbit.
Choose the correct answer from the options given below :
$A$ and $D$ only
$C$ and $D$ only
$B$ and $C$ only
$A$ and $C$ only
Solution
$F =\frac{ Gm _1 m _2}{ r ^2}$
$\Rightarrow F \propto \frac{1}{ r ^2}$
$\Rightarrow F \propto m _1 m _2$
$\Rightarrow \text { This force provides centripetal force and acts }$
towards sun
$\Rightarrow T ^2 \propto a ^3 \text { (Kepler's third law) }$