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:

force_element_volume 0

See also **force_element_volume_factor**, **force_element_volume_geometry**,
and **force_element_volume_time**.

tochnog 2001-09-02