1. The problem statement, all variables and given/known data Problem - A 120-kg mail bag hangs by a vertical rope. A worker then displaces the bag to a position 2.0 m to the side from the original position, always keeping the rope taut. What horizontal force is necessary to hold the bag in the new position ? Note :- We have to use energy balance methods to solve this problem. 2. Relevant equations K(1) + U(G1) + W(other) = K(2) + U(G2) where K(1) and K(2) are kinetic energy at start and end of displacement U(G1) and U(G2) are potential energy at start and end of displacement W(other) is work done by horizontal force in displacing the bag. 3. The attempt at a solution I attempted to calculate the Kinetic energy of the body at start and end of displacement. Then rearranging above equation to .. W(other) = K(2) + U(G2) - K(1) - U(G1) However since the tension in the rope has to be maintained throughout the displacement to the side I assumed that the body would move in an arc. So this means, I guess, that the estimation of the height the body is raised to would be more complicated. I am not sure of the geometry of the displacement and so I think my height estimate is off somewhat. I am also confused about whether the total work involved in raising the body is just the force x distance that the worker produces or a combination of this and the work expended in overcoming gravity + the work the worker expends (and what about the tension in the rope) . Also my calculations give me the Work expended in raising the weight but how then do I convert this to the Force expended .. (what is the relationship here ... is it just W = F x distance ?). I am afraid I am confused as what quantity or combination of quantities I should be using in my calculations and as I said the geometry of the thing is not clear to me. Suffice to say the correct answer is .. 740 Joules .. and I got several answers between 200-400 Joules in my several attempts at the problem. Can anybody help me understand this problem and the general method to use ?