Define impact pressure.

• A. ambient pressure
• B. dynamic pressure
• C. static pressure plus dynamic pressure
• D. total pressure

Answer: (C)

Impact pressure is the pressure the moving air would have if it were brought to rest by the measurement device. That means it represents the total pressure of the flow, which equals the ambient static pressure plus the dynamic pressure carried by the moving air. The dynamic pressure (1/2 ρV^2) is the portion due to motion, and when you combine it with the static pressure, you get the stagnation or total pressure. In a Pitot tube, the air is slowed to zero velocity, converting its kinetic energy into pressure, so the measured impact (stagnation) pressure is Ps + q. That’s why this option—static pressure plus dynamic pressure—best describes impact pressure. The ambient pressure alone isn’t it, dynamic pressure alone isn’t the whole pressure in the flow, and total pressure is essentially the same quantity expressed in a different way.