Q: I have a system composed of several "things". Should I model them as a single body with several parts, or as separate bodies?
REV 5.05

A: Unfortunately, the answer here depends on what you are interested in investigating. First, Bodies are considered to be rigid (except when using generalized coordinates). Thus, if there is to be relative motion between two "things", then they must be separate bodies. Even if there is no relative motion, there are times when you may want to use separate bodies. Suppose that you are interested in the forces which hold two things together. If these two things are parts of a single body, there is no way to "look" at the connecting forces except by doing a structural analysis. If, however, these things are modeled as two bodies connected by some system of connectors, then the connection forces are readily available. Remember, however, that these connection forces are computed assuming the bodies to be rigid, so a stress analysis will yield somewhat different results, depending on the flexibility of the bodies.