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

Question from watching a water fountain today

  1. Nov 27, 2006 #1
    so i was watchign a water fountain during a rally here today and was watchign the fountain in the middle and was wondering why (physically) when the water reaches the top it begins fallign in such a wide pattern. it makes sense to me intuitively but i don't know enough about physics to know just how it works. is it related to bernoulli's equation somehow? THat was my only thought when seeing it that maybe the spread of the water at the top can depend on the diameter of the pipe emitting the water and the pressure with which the water is pumped (and therefore the height it reaches) does any of that make sense? Am i even close to understanding just how this works? I guess that week off made my brain want to be used again so i'm pretty curious now. thanks!
  2. jcsd
  3. Nov 28, 2006 #2
    well obviously it cant fall downwards as more water is coming from there so it must be pushed outwards somewhat as it moves past the water moving up. this gives it a certain initial horizontal velocity which sorta makes it behave like a projectile, or at least you could do a rough model of the motion of those droplets using those equations
  4. Nov 28, 2006 #3
    well yes that part is true, but the stream of water seemed to get wider as the height increased. why is that?
  5. Nov 28, 2006 #4
    I've often wondered how a stream of urine initiates and maintains a uniform flow. No kidding!
  6. Nov 28, 2006 #5


    User Avatar
    Science Advisor
    Homework Helper

    If you start at the origin and roll a ball bunch of balls, one after another, in the x-direction, and then nudge each one in the y-direction a little once it reaches x=5, then the balls farthest from the x-axis will also be the ones farthest from the y-axis. My guess is that the water molecules in the stream don't go exactly straight up, so they undergo glancing collisions with other molecules in the stream, and this gives them a sideways nudge. This nudge gives them sideways momentum, and as they carry on upwards, this momentum carries them farther from the center of the stream as well. So the molecules that are higher up have been in the air longer, and hence have had sideways momentum for a longer time, and hence are further out to the side. That's my guess.
  7. Dec 2, 2006 #6


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    If you're my age it doesn't. I went to a u/r doc last year about my prostate and he said it wasn't that, but rather "mucus in the urethra". He offered no help. Not all the guys who leave spots and puddles in the men's room are doing it deliberately. Would you believe sideways? Yep, a real right angle.
  8. Dec 2, 2006 #7


    User Avatar
    Gold Member

    Everybody's? Rats! I thought I was special. :grumpy:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook