Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What increases when water flows down in a pipe: pressure, speed or both?

  1. Mar 27, 2012 #1

    I am working on a project with a classmate. We have a bucket which represents the water supply and an extensible hose. The bucket is hanged with a rope somewhere high. Here's how it works: the water from the bucket goes down the hose, spins a fan and then gets out of this system.

    We are asked to provide documentation and research, but I don't know where to search for such things.

    Firstly, I don't know what increases, the velocity, the pressure or both? I guess it's the velocity because if we think of two solids dropped from the same height, the second one dropped a short time later than the first one, they would never touch each other, so there's no pressure. But if we consider the friction between the water and hose, then the pressure will increase too. Furthermore, in my case the water flows at an angle, not freely like up-down.

    Can the friction and the force of the water (which hits the fan) be calculated somehow?

    Thanks for reading this!:biggrin:
    Last edited: Mar 27, 2012
  2. jcsd
  3. Mar 27, 2012 #2


    User Avatar
    Science Advisor

    Here are some questions that might help you get started:

    If you drain the entire bucket, using a stopwatch to measure how long it takes drain, and then clock how long it takes for all of that water to arrive at the bottom of the hose, do the times come out different, or the same? Also, if you attach your fan at the top of the hose, rather than the bottom, will it spin at a different speed?

    Hope this helps.
  4. Mar 27, 2012 #3


    User Avatar
    Science Advisor

    The friction in a liquid is parameterized in terms of viscosity.
    For a real world hose, the viscosity is not a negligible effect and should be included. You need to make some assumptions on the boundary between the water and the hose. Perhaps, you can assume that the water next to the hose is stationary (stuck to the hose) and there is a flow gradient as you move toward the center of the hose. You can solve for the flow velocity as a function of pressure using a differential equation based on the viscosity of water. Probably, this has already been done somewhere, so you just need to find a source to cite.

    What level of school are you at? If fluid equations are over your head, then you might be able to treat the hose as a resistor, if the hose is small enough or long enough.
  5. Mar 27, 2012 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    You might start here.


    The second photo shows nicely how pressure and velocity are related.

    Click on the blue "Bernoulli effect" near that picture and it takes you here :
    another graphic showing effect of pressure drop due to friction along your pipe.

    You'll need to transpose your thinking to vertical pipes of course but you'll have no troubles.

    As an experiment, get one of those streamlined garden hose nozzles for the end of your hose, the simple one with non-adjustable tapered snout. They look like a miniature fire hose nozzle and the smaller the better.
    Point it straight up and see how nearly the stream approaches the water level in your bucket. Photograph it with your cellphone for various bucket heights.

    The terms in those links shold enable youto reap rich information via google searches.

    What a great excuse to play in the yard - "It's my physics homework, honest ! ".

    The weather is warming so get wet and have some fun !!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook