For instance, with force as an input, a damper produces a velocity as a consequence. Force sources produce a force, ignoring their velocity. Now we come to transformers.
Transformers serve to transform a force and a velocity into another force and another velocity.
In other words, transformers relate two velocities and two forces. The simplest transformer is the lever . The positive directions of forces and velocities are annotated on the figure. They are two equations characterizing a lever. The second equation is obtained using geometric continuity .