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The concept of impedance of mechanical elements is presented by the analogy to electric circuits. The impedance concept makes the whole field of circuit analysis with its highly developed circuit rules, theorems, and mathematics available for the solution of mechanical problems. Because the calculation of the impedance of most structures is very difficult, a direct measurement of impedance can simplify the problem. Several devices for this measurement are presented. Then, a computer and recorder is shown which calculates and plots impedance information as the measurement is being made. Finally, examples are presented where velocities at points on a simple structure are predicted for sinusoidal excitation and the shock-excited velocity response of a mass mounted on sponge rubber is predicted from impedance information.