In Figure P31.57, the rolling axle, 1.50m long, is pushed along horizontal rails at a constant speed v = 3.00 m/s. A resistor R = 0.400 Ω is connected to the rails at points a and b, directly opposite each other. (The wheels make good electrical contact with the rails, and so the axle, rails, and R form a closed-loop circuit. The only significant resistance in the circuit is R.) A uniform magnetic field B = 0.080 0 T is vertically downward.
(a) Find the induced current I in the resistor.
(b) What horizontal force F is required to keeps the axle rolling at constant speed?
(c) Which end of the resistor, a or b, is at the higher electric potential?
(d) What If? After the axle rolls past the resistor, does the current in R reverse direction? Explain your answer.

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b.