In this section an idealized model of a double pendulum is considered. This pendulum consists of two boxes linked by a hinge. The upper box is lined by a hinge to a non-movable platform, i.e. an inertial system. Both the boxes can move in the XY plane. The corresponding kinematic outline consists of two revolute joints, one frame translation and one inertial system.
The motion of dynamic system cannot be defined just by its kinematic outline, since an outline does not contain mass objects yet. The class Body represents mechanical objects with mass. This class has a connector frame_a as well as the parameters m (mass of the body), rCM (position of center of mass with respect to frame a) and I[3, 3] (inertia tensor with respect to the position of the center of the mass; it's defined by 6 numbers since it's symmetric).
See the Modelica Book, Section 15.10.7.
In this example the pendulum consists of two cylinders. The textual representation of Modelica model is following: