There are two basic types of two-port elements that interest us in modeling mechanical systems—the (mechanical) transformer and the gyrator.
Examples of mechanical transformers are a lever and pulley for translator motions and a meshed pair of gear wheels for rotation.
A gyrator is typically an element that displays gyroscopic properties. These elements can be interpreted as a pair of single port elements whose net power is zero
In this respect the two basic elements are related. The linear graph of a two-port element has two coupled branches. We shall consider only the linear case, that is, ideal transformers and ideal gyrators only.
The extension to the nonlinear case should be clear.
As for a single-port passive element, the arrows on each branch (line segment) of the linear graph give the direction when the product of force and velocity variables for that segment is positive. Note that ui and ƒi are the velocity and force at the input port;