This article studies the problem of applying normal forces using an underactuated aerial vehicle equipped with a two degree-of-freedom SCARA robotic arm. Given known desired location for force exertion, proper initial configuration for the entire aerial manipulation system is computed. The necessary conditions for generating forces along the desired axis are investigated, resulting in exact solutions for the motion of the aerial vehicle and the robot arm joints. A position controller is devised for the vehicle, guaranteeing desired end-effector pose can be achieved during interaction. For the interest of safety, potential fields are used for the contact surface, such that force interaction can only occur under predefined desired conditions. Simulations are presented to demonstrate the force exertion stability and safe recovery against large disturbances.