Dry gas of negligible mass is sealed in a test tube of mass 50 gm with the help of a stopper of mass 3.5 gm. The test tube is suspended from a fixed point with help of massless string such that the test tube is horizontal and distance between point of suspension and centre of mass of test tube is 25 cm. The test tube is heated to a temperature due to which stopper is ejected out horizontally while test tube completes a vertical circle of radius 25 cm. The minimum velocity with which stopper should be ejected out is