A horizontal bar of mass m is free to slide without friction
down the vertical rails of a conducting frame, as shown.
The combined resistance of the bar and the frame
is negligible compared to R, the resistance placed in series
with this circuit.
What is the terminal speed of the bar
as it falls under the influence of gravity
(at the surface of the Earth)
through a uniform horizontal magnetic field
directed perpendicular to the plane of the frame?
(Give your answer in terms of m, B, 
,
R, 
and other fundamental constants.)