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This is homework and I'm having some trouble getting started. How do I go from what's given to the form that they ask for? Normally in something like this I would try to balance torque, but I'm not quite sure how to set it up in this case with voltage.

scan of homework question

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A hint to get you started: $\dot x_c = R \dot \phi_d = \frac{R}{k}\dot \phi_m$ – Alfred Centauri Oct 1 '12 at 19:31
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@AlfredCentauri thanks! that was it, I subbed out phi-dot-m from eqn 1 and substituted i in terms of fc. – mugetsu Oct 1 '12 at 21:33
    
You could post your comment as an answer and then mark the question as answered... :-) – Greenonline Jun 25 '15 at 22:09

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