How Everything Works
How Everything Works How Everything Works

Question 146

In a sealed car driving down the road, when would you have the lowest pressure outside the car: when a window was just a little open, or all the way open? Or would the overall pressure be constant once the window was opened at all?
The pressure outside the closed front windows of a moving car is lower than atmospheric pressure because the air flowing past the car is moving particularly fast as it arcs around the front portions of the car. When you open the front windows of the car slightly, you don't disturb this airflow very much, but you allow air from inside the car to flow outward toward the low-pressure air passing the windows. As a result, the air pressure inside the car drops below atmospheric pressure and you may feel your ears "pop." But if you open the windows wide, the air flowing around the car will probably be seriously disturbed and the low-pressure regions may vanish. As a result, the air pressure inside the car will probably be about atmospheric. However, there are times when the airflow past an open window becomes unstable and the moving air can actually fluctuate in direction, so that it's deflected in and out of the window. When that happens, the whole car begins to act like a giant whistle and you feel the air pressure inside it rise and fall rhythmically. This oscillation is irritating to your ears.

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