Which type of pressure energy is measured only against surfaces that the air directly strikes?

The type of pressure energy that is measured only against surfaces that the air directly strikes is velocity pressure. Velocity pressure is derived from the kinetic energy of the air as it flows, and it is specifically related to the speed of the airflow. When air strikes a surface, the dynamic component of that pressure exerted directly against the surface is what velocity pressure represents.

This concept is vital in applications such as HVAC systems, where understanding airflow and its impact on surfaces is crucial for system efficiency. Velocity pressure reflects the amount of force the moving air exerts when it impacts a surface, making it a critical measurement for ensuring that systems are designed to accommodate and effectively utilize moving air.

In contrast, static pressure refers to the potential energy in a fluid at rest and does not provide information about the flow or movement of air. Dynamic pressure is a more general term that takes into account both static and velocity components, while total pressure is the sum of static and dynamic pressures, measuring the total energy within the fluid. Each of these other types of pressure serves a different purpose and is assessed under distinct circumstances, highlighting why the focus here is on the specific relationship between velocity pressure and the impact of air striking a surface.