Direct approach: regrouping the terms
The preceding equations can be reorganised by regrouping the terms that depend on the same masses, i.e, linked to the same degrees of freedom:
The rewriting shows the existence of three groups: two are linked to one degree of freedom (the terms with
or
).
the third group is common (with a change of sign) to both equations, and describes their coupling (the term
).
The representation of a system will therefore have two groups depending on the degrees of freedom, linked by a third which describes their coupling. Following the type of analogy used, the equivalent network describes these groups by the loops (direct analogy) or the branches (indirect analogy).
