Mukta asked:

“imagine a planetary system in which gravitational force varied as 1/r instead of 1/r² .what relation would correspond to KEPLER’S THIRD LAW (SQUARE OF TIME PERIOD / CUBE OF SEMI MAJOR AXIS.?”

(Answer)

If the gravitational force varied as 1/r instead of 1/r² , then Kepler’s Law of periods would have to be modified accordingly and it would give T proportional to r.