11.2.2 Constraint Reactions

While the use of generalized coordinates has little effect on the application of loads to a model, finding constraint reaction forces is another matter. When a constraint is eliminated from a model by using generalized coordinates, information regarding the reaction forces at the constraint is lost. This is one basic disadvantage of a reduced coordinate system; by algebraically eliminating coordinates from a model we lose information about those coordinates.
Thus, three restrictions are placed on models using generalized coordinates: the Reaction function cannot be used to find the reaction forces applied by a generalized coordinate constraint to any body (because it is not really a constraint), the Reaction function cannot be used to find the reaction forces applied by any constraint to a generalized coordinate slave body (because the body doesn't have Cartesian coordinates), and the Loads function cannot be used to find the loads applied to a slave body (for the same reason).
Both the Reaction and Loads functions may still be used with generalized coordinates, but they must be used to access the coordinates directly.

Alternate usages of Reaction and Loads.

The physical meaning of the quantities returned by Reaction and Loads is quite clear, in the context of the simple generalized coordinate constraints. If the generalized coordinate sym represents an angle in radians, then the loads on the coordinate are in consistent units of moment. If the coordinate sym represents a distance, then the loads on the coordinate are in consistent units of force.
However, if abstract user-defined generalized coordinate constraints are used, the generalized loads may have little physical meaning, just like the generalized loads associated with each constraint (1, 2, ...) sometimes have little physical meaning. The one meaning that always applies to a generalized load is this: a generalized load multiplied by a small variation in its associated generalized coordinate is equal to a small variation in work, in consistent energy units.