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Suppose air is flowing from west to east and door is facing north . Now, I know using Bernoulli's equation the air outside is flowing at higher speed when compared to air inside which creates a pressure difference and that's why doors and windows open.

Now what I don't understand is, if air is flowing from west to east (or parallel to w.r.t doors), why does the door automatically accelerate or move from rest perpendicular to the air.

This is quite opposite to flow of water in a pipe , in which liquid flow in direction to the pressure difference i.e parallel due pressure difference ?

What is the direction of force created by pressure difference?

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  • $\begingroup$ Pressure is not a vector, i.e. you can't assign a particular direction to it. Which implies that if pressure in a region/at a point is high it would apply a force in whichever direction there is a pressure difference. $\endgroup$
    – Gurjot Singh
    Commented Oct 18, 2021 at 3:12
  • $\begingroup$ It's for the same reason of why do pipe's valve opens when water is beginning to flow in it, i.e. dynamic pressure of flow, in this case - wind (air flow) pressure. $\endgroup$
    – Agnius Vasiliauskas
    Commented Jul 8, 2022 at 8:58
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    $\begingroup$ Pressure is a vector field, saying that it can't have a uniform direction is like saying on a rainy day that raindrops doesn't have a direction too. $\endgroup$
    – Agnius Vasiliauskas
    Commented Jul 8, 2022 at 10:53

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When the wind flows, there is a region of low pressure outside the door. This region of low pressure makes the pressure indoors higher and hence the door is flung open. Now that the door is open, the ram pressure exerts a force on the door. When the force exerted by the ram pressure is greater than the force due to the pressure difference, the door closes. You might notice that when it is windy, the doors and windows flutter (open and close in rapid succession) rather than swing open. That is because the two phenomena mentioned above occurs on after another.

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for the west wind case we assume the door faces west i.e., the wind is blowing towards the door. this means the wind is applying a small pressure to the outside of the door which is not balanced by an equal pressure on the inside, thereby creating a pressure difference across the thickness of the door. pressure is in pounds of force per square inch, and multiplying pressure times the number of square inches of door surface exposed to the wind yields pounds of force. That force is trying to pop open the door.

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