Air drag

Because the duct exit is at a lower pressure than the entrance, there is a flow of air from the entrance to the exit. This flow has to be maintained throughout the installation process. Where the flow passes over the cable, it creates a dragging force by interacting with the surface of the cable. The magnitude of the force depends on the surface texture of the cable, the flow speed of the air, and the state of that flow. The flow can be laminar or turbulent, or a range of intermediate conditions. In addition to the drag on the cable, the air flow creates a drag on the duct itself. These two drags, on the cable and on the duct, correspond with an equal and opposite drag on the gas, which tends to impede its progress towards the exit.

The two forces of buoyancy and air drag combine to produce an overall force on the cable towards the exit. This force is distributed over the length of the cable. It tends to be greater towards the exit, because that is where the air flow and pressure gradient are greatest.