question_answer

Which of the following quantities remain constant in a planetary motion (consider elliptical orbits) as seen from the sun?

A)  speed                 

B)  angular speed

C)  kinetic energy                 

D)  angular momentum

Correct Answer: D

Solution :

As force acting on planet is central in nature so torque acting on planet is zero about the satellite & hence its angular momentum remains constant. In elliptical orbit as planet revolves around the sun, the seperation between sun & planet changes & hence moment of inertia which in turn changes the angular velocity to keep angular momentum constant. \[K=\frac{{{L}^{2}}}{2I}\]as I changes, K also changes.