1. The problem statement, all variables and given/known data this is from an example problem in the hibbler dynamics book 13th edition in the section of work so i dont really have a question on how to do it. the question is why is the problem solved the way it is solved. the system has one pulley.one rope.and two masses at each end of the rope the work done by gravity is positive for both masses....why?. since one mass is going up and the other mass down. shouldn't work by gravity be negative for the mass going up? i assign positive going down. 2. Relevant equations work done on an object = the change in the objects KE 3. The attempt at a solution i solved with kinematic equations and found the answers match. so I know its correct to assume that both masses go down with energy equations, but if I try to solve with one mass going up (and so work by gravity is negative on that one)and the other down (so work by gravity is positive on this one) using energy equations I get a different/wrong answer. so why must both masses be assumed to move down? is it because of the rope? How? also, tension does zero work. Why? is it because its a reaction force? can reaction forces do work?