More Track and Field

A recent item in Physics Challenges for Teachers and Students 1 presented a problem of a metal rod (mass M) sliding on frictionless, parallel conducting rails (of negligible resistance and separated by a distance L) in the presence of a downwardly directed uniform external magnetic field B. A resistor R bridges the rails, thereby completing the circuit. This problem can also be found in Griffiths' text Introduction to Electrodynamics, 3rd ed. (Problem 7.7).2