A metal rod having a mass m and length l has resistance R. The rod hangs from two vertical metallic wires of length l in a uniform vertical magnetic field B of a horseshoe magnet (see figure). The wires have negligible resistance. An AC voltage source is connected between the wires which provides a voltage U(t) = U0 cos ωt, where U0 and ω are constants. a) Show that for small angular displacements φ the motion of the rod can be described by the equation U(t) = αφ ̈ + βφ ̇ + γφ, and express the constants α, β, γ in terms of quantities m, l, B, R and the gravitational acceleration g. b) At a certain frequency ω = ω0 the amplitude of the oscillations of the rod becomes large and the equation above does not describe the motion anymore. Estimate the frequency ω0.
A metal rod having a mass m and length l has resistance R. The rod hangs from two vertical metallic wires of length l in a uniform vertical magnetic field B of a horseshoe magnet (see figure). The wires have negligible resistance. An AC voltage source is connected between the wires which provides a voltage
U(t) = U0 cos ωt, where U0 and ω are constants.
a) Show that for small angular displacements φ the motion of the rod can be described by the equation
U(t) = αφ ̈ + βφ ̇ + γφ,
and express the constants α, β, γ in terms of quantities m, l, B, R and the gravitational acceleration g.
b) At a certain frequency ω = ω0 the amplitude of the oscillations of the rod becomes large and the equation above does not describe the
motion anymore. Estimate the frequency ω0.

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