Using the simulation....

SW uses inertia, but it does simulation in a very simplified way (and often gives surprising results) CosmosMotion and other "real" motion simulation packages do such things much better. However, models involving changing contacts, such as a part rolling on another, or gears are always difficult, if not impossible to simulate correctly because the contact forces are computed in a simplified way (i.e do not consider material deformation)
motion simulation packages (and SW2004) let you insert linear springs in your model/simulation. If you need to compute the deformation of a part subject to its own weight or to external forces, you'll need a finite elements analysis (FEA) add-in such as CosmosWorks, or MSC/VisualNastran or one of the many others.