A uniform slender bar of length is initially vertical and…

A unifоrm slender bаr оf length is initiаlly verticаl and at rest. Its upper end A is attached tо a small roller constrained to move along a smooth horizontal surface (i.e., no friction). A horizontal force of  is suddenly applied to the roller as shown. Neglect the mass and size of the roller.  What is the angular acceleration of the bar immediately after the force is applied?