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Just a quick fluids question.

  1. Dec 14, 2003 #1
    What's the difference between a fluid's flow rate and flow speed? I use the continuity equation to get the flow speed I thought but I couldn't think of how to get the rate. Here's the exact question if you want that for reference:

    A liquid of density equal to 1.85 g/cm^3 flows through two horizontal sections of tubing joined end to end. In the first section (large section) the cross-sectional area is 10 cm^2, the flow speed is 275 cm/s, and the pressure is 1.2E5 Pa. In the second section (small section), the diameter of the tube is 2.5 cm.

    -Find the flow speed, flow rate, and the pressure in the small tube section.

    Thank you for your time and help!

    -edge
     
  2. jcsd
  3. Dec 14, 2003 #2

    Doc Al

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    Staff: Mentor

    I believe flow speed is the speed of the fluid at some point, while flow rate is the quantity of fluid moving past a certain point. Flow rate depends on cross sectional area.
     
  4. Dec 14, 2003 #3
    Thanks.

    Ahh, I can see the difference now...but no where in my physics book can I find an example of this. Is there an equation for finding it? Or does it involve some manipulation of the continuity or bernoulli's equation?
     
  5. Dec 14, 2003 #4

    Doc Al

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    Re: Thanks.

    The continuity equation is the flow rate equation! VA = the flow rate (volume per time). The continuity equation says that the flow rate is constant.
     
  6. Dec 14, 2003 #5
    Ahh silly me. This I see. But I still had points taken off (this is from a previous test), I guess for not having a flow speed. How exactly would I be able to find that with the given information? I'm at a loss :( just trying to figure it out before the final tomorrow. Fluids are the only thing that still gives me trouble =x Well, and some harmonic motion problems. So if you could point me in the right direction, I'd greatly appreciate it! Thanks again for your help.

    -edge
     
  7. Dec 14, 2003 #6

    Doc Al

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    You are given the speed (275 cm/s) and cross-sectional area (10 cm^2). Go forth and multiply.
     
  8. Dec 14, 2003 #7
    Yeah, I figured it out. Sorry for not responding soon enough. I was too used to using AV1 = AV2 to realize that AV is the flow rate. I just plugged in the numbers to get the flow speed in the smaller pipe and then multiplied my new speed with the crossectional area (found with the given diameter). Sorry for the trouble and my ignorance. Too much studying = sore brain. Thanks again.

    -edge
     
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