atwood machine and induction

[silstar]

hi ! i found a problem in english and it is very difficult
can you help me?
A conducting bar of mass m and resistance R is pulled in the horizontal direction across two frictionless parallel rails a distance l apart by a massless string which passes over a frictionless pulley and is connected to a block of mass M, . A uniform magnetic field is applied vertically upward. The bar is released from rest.
Let the speed of the bar at some instant be v. Find an expression for the induced current. Which direction does it flow, from a to b or b to a? You may ignore the friction between the bar and the rails.
thanks

[Camillo]

You should at least show some tentative solution and then explain where you have difficulties.