Atwood's Machine, with one pulley and two weights, is analyzed with two free body diagrams. Each diagram depicts the forces applied to each weight. If the weights are connected, why isn't gravity considered to be pulling on both weights in the same direction? (Meaning, why aren't the masses for the weights combined to make one force diagram with one mass? Because the acceleration is not equal to 9.8m/s/s?) A simpler question may be how does an Atwood Machine work?