Question

Q. In the video of gravitation chapter, in the third law of Kepler's, it is said that, the square of the time period of a revolution for a pair of planets is a directly proportional to the cube of the semi major axis. But in our NCERT textbook it is given for a planet. Is this law is applicable for a pair of planets or for a single planet ?​

Answer 1

The minimum condition for a satellite to orbit around a planet is a circular orbit and let it's velocity be v .The minimum velocity for escaping the planet is v(e).For velocities in between ,the satellite will stay in the planet's orbit with an elliptical path.In the textbook it is given for sun and planet system ,where the reference frame taken is sun at rest.Here planet can be taken as satellite of sun.
In similar case ,for two planetary system,one can be taken as satellite of another planet.So the kepler's law is applicable in both cases.