The unbalance of the rotor of a 400-lb motor is equivalent to a 3-oz weight located 6 in. from the axis of rotation. In order to limit to 0.2 lb the amplitude of the fluctuating force exerted on the foundation when the motor is run at speeds of 100 rpm and above, a pad is to be placed between the motor and the foundation. Determine 

(a) The maximum allowable spring constant k of the pad, 

(b) Tthe corresponding amplitude of the fluctuating force exerted on the foundation when the motor is run at 200 rpm.

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Fig. P19.118