Motivated by the relation between a force acting on a flexible arm and the resultant bending deflection, this paper presents a force control technique simply via position control. The dynamic equations of a flexible arm with tip position placed on a fixed horizontal plane are modeled by an approach similar to the finite element method. Based on this model, the constraint force on the horizontal plane can be directly estimated from the motor's angle and the bending deflection, and thus the use of force sensor is no longer needed. The controller structure consists of two nested feedback loops in which the inner loop is designed for the motor's angle by a PD control algorithm and the outer loop deals with the constraint force by sliding-mode control. Experimental results are included to demonstrate the success of the developed controller.