Distributed volume source terms (like force, heat generation, etc.) in differential equations. Here force_0 is the source term for the first primary unknown, etc. Use the control_print data item to print dof_label to get the sequential order of the primary unknowns. Specify source terms for each principal unknown which is present in the calculation (see the start of the data part description for a list of principal unknowns). Consider the example with distributed volume force in x-direction for a 2D material:
See also force_element_volume_factor, force_element_volume_geometry, and force_element_volume_time.