Suspended Ring - Masses Released - Find Condition for Ring to Move Up

[Chileboy]

I'm having trouble with this problem...

A ring (mass = M) is suspended by an ideal cord from the ceiling, two equal masses (mass = m) are released from the top of the ring and slide down, one on either side of the ring, without friction. The question is:
What condition must the masses meet in order for the ring to move up?

I'm having trouble imagining it, I can't see which force could make the ring go up.

I'd apreciate any help.

 

[spacetime]

Ring and masses

As far as I see it, as the masses reach the lower half of the ring, there is a normal reaction acting on them exerted by the ring. But that is in the upward direction.

But after they collide at the bottom, they ( if they are elastic ) start moving up again and in the process, the normal recation exerted by the ring is downwards, and on the ring by the masses is upwards. Probably that is what makes it go up.


spacetime
http://www.geocities.com/physics_all/index.html