This information is part of the Modelica Standard Library maintained by the Modelica Association.

The variables of a component of this library can be accessed in the usual way. However, since most of these variables are basically elements of vectors, i.e., have a direction, the question arises how the signs of variables shall be interpreted. The basic idea is explained at hand of the following figure:

First, one has to define a positive direction of this line, called axis of movement. In the top of the figure this is characterized by an arrow and a corresponding text. The simple rule is now: If a variable of a component is positive and can be interpreted as the element of a vector (e.g., force or velocity vector), the corresponding vector is directed into the positive direction of the axis of movement. In the following figure, the right-most mass of the figure above is displayed with the positive vector direction displayed according to this rule:

The cut-force mass2.flange_a.f of the right mass is directed into the direction of movement if the values are positive. Similarly, the velocity mass2.v of the right mass is also directed into the direction of movement if the values are positive