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General Help in Understanding Pressure. PLEASE HELP!

  1. Sep 30, 2011 #1
    I thought I understood pressure in fluids, but it seems like its starting to make less sense to me.

    Let's consider a flow through a pipe where the rho g h term drops out. Therefore, we would only have the dynamic and static pressure. This is where I start to lose it. I have been told that when the flow of water goes from a wider section of a pipe to skinnier one. the dynamic pressure increases and static pressure decreases. The dynamic pressure going up makes sense, but I don't understand why the static pressure goes down. I think it may be that I don't understand exactly what static pressure is.

    Does the static pressure act equally in all three dimensional directions or does it act only horizontally and vertically? Does it act on the pipes pressing outward or does it act horizontally pressing the fluid forward? I don't understand why the pressure wouldn't increase if the fluid is moving faster because i think getting hit by something fast would hurt more than getting hit by something slow.

    Could someone please give a good basic description of what it is and how it changes because I can't find any simple answers. I would really appreciate it!
  2. jcsd
  3. Sep 30, 2011 #2
    To add to that, if you're using a manometer, why would the height change if the total pressure is constant throughout the entire flow?
  4. Sep 30, 2011 #3
  5. Oct 1, 2011 #4


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    It's energy balance.

    You could compare it to dropping a ball from a height h.
    At h the ball has a potential energy but no kinetic energy.
    from the moment the ball is dropped, the potential energy decreases but since the ball is picking up speed, the kinetic energy increases.

    Or a pendulum with rising and falling potential and kinetic energy as the bob sweeps along its arc.

    The static and dynamic pressure work the same way to balance the energy equation.
    As the diameter of the pipe cahnges from small to big or visa versa the dynamic and static pressures change also.

    You get hurt from a fast moving stream of water because the water is being stopped by a particular small part of your body, and all that energy concentrated in one spot. A slow moving stream of water of the same amount of energy could possibly sweep you off your feet.
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