Masses on Pullies, equal weights on both sides but still rotating

[Oijl]

Homework Statement

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.

Homework Equations

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?

 

[rl.bhat]

In the second pulley, m is going up and 3 m is going down. If net force on m is f1 and that on 3m is f2, the net force on the second pulley is f2 - f1. And f1 and f2 are not the weights. So 4mg on the right is greater than f2 - f1. Hence the first pulley rotates.