A horizontal plane supports a plank with a bar of mass m placed on it attached by a light elastic non deformed cord of length L  to a point O.   The coefficient of friction between the bar and the plank is k=.20.   The plank is slowly shifted to thje right until the bar starts sliding over it .  It occurs at the moment when the cord deviates from the vertical at an angle @=30.  find the work that has been performed by that moment by the frictional force acting on the bar in the reference frame fixed to the plane.