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An electromagnetic fatigue-testing machine is shown in Fig. 3.41 in which an alternating force is applied to the specimen by passing an alternating current of frequency f through the armature. If the weight of the armature is 40 lb, the stiffness of the spring (k1) is 10,217.0296 lb/in,. and the stiffness of the steel specimen is 75 x 104 lb/in., determine the frequency of the alternating current that induces a stress in the specimen that is twice the amount generated by the magnets.