Considers a mechanical device defined with an input gate and an output gate. At the input gate there are two values of the force and the velocity . At the output gate there are the two values of the force and the velocity . So we have all in all two gates and four items. This system can be described in the equivalent manner as an electrical quadripole. So we call this system a mechanical quadripole. We can consider it as a black box. It is not important of what the device consists. It is only important how it works. In this worksheet we will see how we can use this method to work with some special dynamical systems. But first we have to give some conditions for the mechanical systems. Perhaps not all of the following conditions are necessary, but they will help to understand this idea.