A wooden core (Fig. 4.36) supports two coils: coil 1 with inductance L1 and short-circuited coil 2

Question:

A wooden core (Fig. 4.36) supports two coils: coil 1 with inductance L1 and short-circuited coil 2 with active resistance R and inductance L 2. The mutual inductance of the coils depends the distance x between them according to the law L12 (x). Find the mean (averaged over time) value of the interaction force between the coils when coil 1 carries an alternating current 11 = Io cos cot.