A 125kg mail bag hangs by a vertical rope 3.9 m long. A postal worker then displaces the bag to a position 1.9 m sideways from its original position, always keeping the rope taut. What horizontal force is necessary to hold the bag in the new position? I used the gravitational potential energy formula: mgy1-mgy2 and got 2450 but that is not the right answer. And I do not understand what they mean by moving it sidways. Is that why I am getting the answer wrong?