During gas of negligible mass is sealed in a test tube of mass `50 gm` with the helo 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