These are forces which act in the same plane (coplanar, not coplanner!) and that their lines of action all meet at a single point.
Coplanar (not coplanner) forces are those that act in the same plane and, if they are parallel, then their lines of action are parallel: that is, the perpendicular distance between the lines of action is a constant.
The answer depends on whether you mean A intersecting (B union C) or (A intersecting B) union C.
"cutting across"
If it is on a graph, intersecting means two or more lines go through the same point.
Often the pilot might say aircraft intersecting basically it means plane landing. So when the pilot says intersecting it tells the crew to be in possessions for landing.
there are no outside affecting forces.
It means they cross each other.
It's when two lines cross eachother
This means that the angle of intersection is not 90o.
Damp oscillation system means the external forces are playing their role as drag force , air resistance , gravity etc. These forces causes to stop the motion of oscillation.