The concept of impedance extends also to masses and springs. Figure 7.2 illustrates an ideal mass of kilograms sliding on a frictionless surface. From Newton's second law of motion, we know force equals mass times acceleration, or
Since impedance is defined in terms of force and velocity, we will prefer the form . By the differentiation theorem for Laplace transforms ,8.1we have
Since we normally think of an applied force as an input and the resulting velocity as an output, the corresponding transfer function is . The system diagram for this view is shown in Fig. 7.3.
The impulse response of a mass, for a force input and velocity output, is defined as the inverse Laplace transform of the transfer function:
Once the input and output signal are defined, a transfer function is defined, and therefore a frequency response is defined . The frequency response is given by the transfer function evaluated on the axis in the plane, i.e., for . For the ideal mass, the force-to-velocity frequency response isamplitude response rolls off dB per octave, and the phase shift is radians at all frequencies.
In circuit theory, the element analogous to the mass is the inductor, characterized by , or . In an analog equivalent circuit, a mass can be represented using an inductor with value .