atwood machine problem -- symbolically solving for mass? 1. The problem statement, all variables and given/known data Symbolically solve the equation derived for the acceleration of the Atwood's machine in part 1, A-3 for the mass m in terms of M, g, and the acceleration, a. (No numbers.) This question is part of a lab, so of course some other information is necessary. We were finding theoretical and experimental acceleration for an Atwood's machine when the masses on both sides were the same except for a few pennies balanced on one of the masses. After finding this value, we were using it to fine the theoretical and experimental weights of the pennies. We were distinguishing m from M: m was the much smaller mass of the pennies, and M was the mass of the weights on both sides. 2. Relevant equations How I solved for acceleration can be found in attached image #1. The basic equation used is Newton's second law. 3. The attempt at a solution My weak attempt at solving for mass is in the second attached image. It all seems to cancel out and I'm getting really confused. Can anyone tell me where I made a mistake, or whether I'm just using really faulty, circular logic?