I've read a couple other threads about this issue. And one even addressed my exact question but the answer didn't help me understand this at all. In an Atwood machine, why isn't the total force on a mass = its gravity minus the gravity of the mass on the other side? I keep seeing tension in every example online as being equal for both but it seems to me like the tension of the string touching a mass should be equal to the weight of the other mass. I've tried to model the forces. I see each weight has gravity weight and an upward force caused by the weight of the other mass. Yet nothing I see does it this way or gets answers anything like that.