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Homework Help: Drop in Pressure Over A Given Distance

  1. Nov 9, 2004 #1
    Hey I've been working on this pressure problem, and I am really stumped. We just started fluids, and I'm not really sure where to start. If someone could help me or point me in the right direction, that'd be great. Thanks! Heres the problem:

    A straight horizontal garden hose 43.0 m long with an interior diameter of 1.50 cm) is used to deliver 20C water at the rate of 0.510 liters/s. Assuming that Poiseuille's Law applies, estimate the pressure drop (in Pa) from one end of the hose to the other.

    Like I said, I'm not even sure where to start. :confused:
  2. jcsd
  3. Nov 11, 2004 #2
    Ok, I've been working on this and I think I have some progress, and I'm so close but I just seem to be missing a key point or something. Using Poiseuille's Law:

    Now to find n , the coefficient of viscoscity, i have to use the equation :

    F(visc)=n *(Av)/D

    where A is the cross-sectional are of the pipe, and v is the velocity of the fluid.

    So first, how do I calculate n if I don't have the internal friction, which I labeled as F(visc)? Or do I have that number? Im so confused.

    And also, in Poiseuille's law, where do I get my initial and final pressures? P1 and P2.

    This is what I've gotten so far, and I think I'm really close, but I just need n , correct? Thanks.

    Edit: And how do I use whatever it is people on this board use to make the actual symbols, etc? It would make it alot easier to write out those equations. :biggrin:
  4. Nov 11, 2004 #3


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    The coefficient of viscosity [tex]\mu[/tex] is assumed to be known. It is a physical property. Moreover, you've got all the data necessary to solve the problem merely by substituting them into the Poiseuille formulae. You don't need the pressure itself, but the pressure difference is required and it's what you work out in the Poiseuille expression.
  5. Nov 12, 2004 #4
    Yeah thats what I was assuming, however as you can see its not included with the problem. What forumla can I use to calculate it using the information I'm given? :confused:
  6. Nov 13, 2004 #5


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    [tex] \mu_{water}=1.12\cdot 10^{-3} Ns/m^2[/tex]

    I was referring to Poiseuille Law.
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