The "Mechanisms" group is meant to cover the MDX & MDO tools in Pro/Engineer or Creo CAD software. This tool is very different from using Mathcad as a way to model the kinematic and dynamic behavior of mechanisms. Using Mathcad requires detailed understanding of the equations that govern the behavior and requires much more work to construct representations of systems compared to a CAD package that offers this functionality. It's best to have these topics kept separate since the work required for each is much different. If you have access to Pro/E (Creo) then the "Mechanisms" group is best. If you have access only to Mathcad then the other two groups listed above would be excellent to join and participate in. I happen use both and look forward to the day when Mathcad will hopefully be tightly integrated to MDO to define more complicated loading scenarios.