question_answer

A very long (length L) cylindrical galaxy is made of uniformly distributed mass and has radius R (R << L). A star outside the galaxy is orbiting the galaxy in a plane perpendicular to the galaxy and passing through its centre. If the time period of star is T and its distance from the galaxy’s axis is r, then:                                        [JEE ONLINE 10-04-2015]

A) \[T\propto {{r}^{2}}\]  

B)      \[T\propto r\]

C) \[{{T}^{2}}\propto {{r}^{3}}\]          

D)      \[T\propto \sqrt{r}\]

Correct Answer: B

Solution :

[b] k is some constant so constant                       straight line