What is the explanation for the 4m mass going down in a double Atwood machine?

[player1_1_1]

Homework Statement

I have a double Atwood machine; on left side there is a heavy thing which weight 4m, on right side there is another circle (this circle which go around in atwood machine) and on left there is heavy thing which weight m, on right - mass which weight 3m. Now the 3m thing starts to go down, m thing goes up. Why the 4m mass goes down, while the mass on both sides are the same mass?
http://img10.imageshack.us/img10/9270/testttti.jpg

Homework Equations

I got this result from Lagrange equations

The Attempt at a Solution

I tried to research how the forces in lines and masses are existing, but I couldn't find any soluton for this.. why this 4m mass is going down? thanks for answer!

 

[tiny-tim]

.. why this 4m mass is going down?

Hi player1_1_1!

If the 4m didn't move, then the centre of mass would go down.

So the 4m must go down to keep the centre of mass where it should be.

(you'll probably find that 4a + b + 3c is a separable variable)

 

[player1_1_1]

Thanks for answer!
does it have any explanation why centre of mass must be in same location? maybe some additional forces which force this 4m thing to go down?

 

[tiny-tim]

Newton's first law (and zero net external force) ?

(Imagine you're hanging from the right-hand disc, instead of the masses, and you put your arms "above" your head … would you expect the left-hand mass to stay where it is?)

 

[player1_1_1]

now i understand this, thanks!