Impeding force - friction

Of course these are not the only forces acting on the cable. There are other forces that occur where the external surface of the cable and the internal surface of the duct interfere with one another. One of the most important of these is the frictional force. It opposes the aerodynamic forces and tends to prevent the cable proceeding towards the exit. The frictional force tends to be much larger in bends in the duct than elsewhere. This is generally referred to as "the capstan effect".