1. The problem statement, all variables and given/known data Consider a double Atwood machine constructed as follows: A mass 4m is suspended from a string that passes over a massless pulley on frictionless bearings. The other end of this string supports as second similar pulley, over which passes a second string supporting a mass of 3m at one end and m at the other. Find the acceleration of the mass 4m when the system is released. Explain why the top pulley rotates even though it carries equal eights on each side. 2. Relevant equations 3. The attempt at a solution The mass 4m accelerates downwards with an acceleration of g/7. Can anyone explain why, though, the pulley rotates at all, since it has equal weights on both sides?