def surface_force(component=None, force_variables=None,
cg=(0, 0, 0)):
''' Compute the forces for a surface. Returns the forces, and
moment about cg, and area for each component.
optional arguments:
components - (Component)
force_variables - (list of str)
cg - (list of float) (1-by-3)
'''
global _dataset
if component is not None:
assert isinstance(component, Component)
zonelist = component.zonelist
else:
component = Component('All')
zonelist = list(_dataset.zones())
if force_variables is not None:
for v in force_variables:
_dataset.add_variable(v)
for zone in zonelist:
pres, fric, mmnt, area = zonal_forces(zone, cg)
component.force_pressure[:] = component.force_pressure[:] + pres[:]
component.force_friction[:] = component.force_friction[:] + fric[:]
component.area = component.area + area
component.moment = component.moment + mmnt
return component
