Why is one Tension Negative and the Other Positive -- Atwood Machines Hello, Please consider the following: I am trying to understand how to solve for acceleration when dealing with Atwood machines. I keep getting hung up on one part: the direction of tension. In the image above, why is tension in the sum of the forces for m2 negative when if I were to draw a force diagram it is pointing up. Furthermore, why is gravity positive? Tension is pulling the masses up and gravity is pulling them down. A force diagram's directions are not different between m1 and m2, so why are they different when summing the equations. Many Thanks.