The double-excited electromechanical system shown in Eigure P14.35 moves horizontally. Assume that resistance, magnetic leakage, and fringing are negligible; the permeability of the core is very large; and the cross section of the structure is $w \times w$. Find:
a. The reluctance of the magnetic circuit.
b. The magnetic energy stored in the air gap.
c. The force on the movable part as a function of its position.