How can the rubbing surface of an airway be calculated?

To determine the rubbing surface of an airway, the calculation involves understanding the surface area in contact with the air flowing through it. The correct approach is to multiply the length of the airway by its perimeter.

This method effectively captures the total surface area that interacts with the airflow, considering how the perimeter defines the boundary of the cross-section of the airway. The length provides the extent over which this perimeter is stretched, thus giving a complete view of the rubbing surface area.

In the context of airways, maximizing the efficiency of airflow while minimizing resistance to movement is crucial, which is why calculating this rubbing surface accurately is essential for engineers and foremen in the mining industry. The other choices, while related to dimensions, do not accurately represent the requirements for calculating the rubbing surface. For example, multiplying area by perimeter doesn't yield a relevant metric for surface contact, and perimeter multiplied by velocity addresses airflow but not the surface area itself.