Say I have a double Atwood machine. A 12kg mass hangs from one side of the first pulley and a second pulley hangs to the other side. This pulley has two masses hanging from it. They are 8kg and 4kg respectively. I originally approached this problem by considering the 8kg and 4kg masses as a single mass relative to the 12kg mass. This lead me to conclude that the system would be in static equilibrium. However the solution in my book involves treating each body separately and deduces that the whole system accelerates relative to the ground. I do not know a lot of physics (did not complete high school). Is this net acceleration as result of torque?