A can filled with water is revolved in a vertical circle of radius 4 metre and the water does not fall down. The time period of revolution will be

 the top most position

three forces will act on the water

1 weight in the downward direction

2  normal reaction by bucket in the downward direction

3 centrifugal force in the upward direction

 so  mg + N =mv²/r

 for water just does not fall   N ≥ 0

so  mg = mv²/r

 v =√gr

so time period T = 2∏r/v

                         =2*3.14*4 /√4g

                          =3.14*4 /√9.8

                         =4.01 sec