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QUESTION: fluid flow from a  pipe with vary cross section;according to equation of contnuity area increases velocity decreases and according bernoulli theorem velocity increses pressure decreases ata a same horizontal pipe but practically we feel a greater pressure when we take their at the narrow section.why.

ANSWER: We don't, actually.  You feel the pressure buildup from the supply, you're thinking of your thumb on the end of a garden hose or something.  Your thumb is exerting force to squeeze the water into a different flow configuration.  Pressure in the whole hose rises tremendously on the sidewall.  While you may feel the force of deflecting the momentum forward (of course), you won't feel a separation force (horizontal, not along the hose axis).  It's counterintuitive, but if you put gauges to measure actual pressures then you'll see (I've set up the equipment, it's quite common in engineering schools) that it's absolutely along the lines of Bernoulli's principle.

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QUESTION: can you explain it more ,i don't understand

Not really.  I'm just saying that you feel a pressure along the axis of the tube that's greater at the narrow section, because you're forming a blockage.  If you cut a hole in the slow section of tube (the wide section) and an equal small (much smaller than the diameters of the tubes at those points) and looked at the pressure out of the SIDE of the tube, water would spray out faster at the wide/slow section.  Your intuition about feeling a greater pressure at the narrow section is just wrong.  I can't explain your intuition, and that's the thing that's wrong here.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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