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If the pressure at the exit of a pipe into the atmosphere equals the atmospheric pressure why do we feel the air? I mean, we don't feel anything at common atmospheric pressure, where is it different ?

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The air exiting the pipe has a velocity, which it got by falling through a pressure differential before getting to the exit.

Thus, it is a wind. When you put your hand in front of it, you stop it, which results in pressure, the pressure needed to stop it, called stagnation pressure.

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  • $\begingroup$ But shouldn't pressure be the force per unit area that a body which is immersed in the fluid will feel? In this case, my hand is the body, so shouldn't it feel the fluid pressure, which is the atmospheric? Or when we say that the flow outside the pipe is at the same pressure as the atmosphere, do we mean the static pressure, or total pressure? $\endgroup$ Oct 18, 2017 at 9:46
  • $\begingroup$ @GeorgeSailor: You feel pressure difference, not pressure. You could go 100 meters underwater, where the absolute pressure is high, but you don't feel that, because it's the same all around you and cancels out. But if there's a flow, as from a water jet, you feel it, because if you put your hand in front of it, you have to provide the extra pressure to stop it, because it can't go through your hand. $\endgroup$ Oct 18, 2017 at 12:15
  • $\begingroup$ But then again, if there's a pressure difference between my hand and the flow so that I feel a pressure force, it means that one of the sides of my hand has a different pressure value, but both sides are in contact with the atmosphere! $\endgroup$ Oct 18, 2017 at 21:40
  • $\begingroup$ @GeorgeSailor: A lot of people think air has no weight, so stopping it doesn't take any force. Not so. You know how an airplane tells its speed? It has a pitot tube, which is a tube with two holes, one hole pointing into the wind, and one on the side. The one pointing into the wind feels a higher pressure than the one on the side. Those go to a bellows chamber. the chamber gets the side pressure, and the bellows gets the front pressure. Since the front pressure is higher, the bellows expands, turning the airspeed meter. Simple, right? $\endgroup$ Oct 18, 2017 at 23:08

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