The term force generically refers to both applied forces and applied moments.  Force elements are commonly used for:

Examples of compliant connections include springs, dampers, and bushings.  For the purpose of vehicle handling studies, tires are modeled as compliant force models.  Examples of environmental effects include gravity (which may or may not be constant), aerodynamic forces, and hydrodynamic pressure.

Force elements may be defined as arbitrary functions of any state variables in the system and time.  The functional form of the force description may be an interpolated curve, an analytical expression, or a user-defined subroutine in C++, C, or FORTRAN.  Vector forces can be specified by their components in arbitrary reference frames.  Scalar forces act along the line connecting the origins of markers.

This is an extremely open and powerful capability in MotionSolve that enables you to easily model most of the forces occurring mechanically.  However, when modeling a force, you must make sure that the force is, at the very minimum, continuous.  It is better if the forcing functions are smooth (without kinks) and do not change dramatically with respect to the independent variables (frequency content is not very high).  If the force has an undamped, high frequency content, the integrator will attempt to follow it and therefore take smaller step sizes, resulting in slow run times.