Illustration of the FEM theory with the 2 nodes bar element.
The simplest finite element is the 2 nodes bar element. It has 2 extremities, called "nodes" by which
it can be connected to other finite elements or supports, and can only shorten or extend, that means
that the unknowns at the nodes are the axial displacements u1 and u2. These are called the degrees of
freedom of the element. Read this topic