Solving a problem in physics section 39 paragraph 24 el

24. two smooth copper bars set at an angle α to the horizontal slides by gravity constant velocity c copper jumper mass m. Tires are closed on the resistance R. The distance between the tires L. The system is in a uniform magnetic field induction B perpendicular to the plane in which the bridge moves. Resistance tires, jumpers and sliding contacts, as well as the self-inductance of the circuit is negligible. Find the speed jumpers.