I dislike simply having answers told to me, so I won't give the actual number values of the question. However, I need some insight on how I would solve this: There are two masses on an atwood's machine, but only one of the masses is known (let's say 5 kilograms). I am told that the unknown mass is greater than the known mass, and that the system is accelerating at 2 m/s^2. I am also told that friction is negligible, and to assume that the string is weightless. How would I solve for the unknown mass? edit: I think it came to me: 2(5) + 2(x) = 9.81(x) -9.81(5) 2(5) + 9.81(5) = 9.81(x) - 2(x) 10 + 49.05 = 7.81(x) 59.05/7.81 = (x) = 7.56kg Is that right?