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

Down the drain

  1. Apr 14, 2005 #1
    I'm sure this has been discussed too many times, but it still seems to be controversial. So, if anyone south of the equator is reading this, please let some water go down the drain (a sink or bathtub, not a toilet) & tell us if it swirls clockwise or counterclockwise.
  2. jcsd
  3. Apr 14, 2005 #2
    I have actually done this before. Crossing the equator at 0/0 onboard a Navy ship I tried this and it went straight down.
  4. Apr 14, 2005 #3
    So at the equator it went straight down. But what about BELOW the equator?
  5. Apr 14, 2005 #4
    Hmm, I don't remember checking that. I was there for about a month and relatively close to the equator for that month. I didn't have any dyes when I tested it but the water level did seem to drop without any rotation.
  6. Apr 14, 2005 #5
    Last edited by a moderator: May 2, 2017
  7. Apr 14, 2005 #6


    User Avatar
    Science Advisor
    Gold Member

    Technically, yes it works. In practice, there are so many other factors that really, the vortex can occur in either direction in either hemisphere.

    Things like drain geometry and currents in the water have more of a role in this than the Coriolis effect.
  8. Apr 14, 2005 #7


    User Avatar
    Gold Member

    OK look. Let's debunk this by trying to prove it.

    Let's pick up a toilet bowl of water and rotate it at the right speed to exactly cancel out the Coriolis Effect. We'll make this an ideal experiment, eliminating all other variables that will throw off our measurement.

    Ready? Go. OK, don't turn yet. I want you to turn slow enough that, after AN HOUR of holding the toilet bowl, you've only rotated 15 degrees. On a 1 foot diameter bowl, that's a movement of about 4cm. 4cm in an hour.

    How long does a flush take? 5 seconds? How long does it take to to impart a direction on that flush to send it one way or the other? 2 seconds?

    So. In 2 seconds, you will rotate the bowl 2/86,400ths of a full turn, or 8 1000ths of a degree, or 2 100ths of a millimeter or 1/5th of the width of a human hair.

    The strongest Coriolis force acting on the bowl - that would be at the pole - is equivalent to rotating the bowl a distance of 1/5 of the width of a human hair during the flush.

    You tell me. Is that enough?

  9. Apr 14, 2005 #8


    User Avatar
    Science Advisor
    Homework Helper

    I believe I did an experiment on this in my freshman year.
    The direction the vortex whirls depends highly on the initial condition of the water, if you give it a very small swirl clockwise or counterclockwise and drain it, it will turn in the respective direction. It's even possible that it doesn't swirl at all (for small enough systems).
    The Coriolis force is way too weak to influence buckets or bathtubs, but on a geographic scale, it can be influentual. I believe the direction in which tornado's swirl is determined by their location on the earth.
  10. Apr 14, 2005 #9


    User Avatar
    Gold Member

    If he'd tried it below the equator, he would have drowned. :tongue:
  11. Apr 14, 2005 #10
    In all honesty it's all going to be affected much more by the way your drain is cut then anything else. That goes both for Northern and Southern hemispheres.
  12. Apr 14, 2005 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yeah, pretty much if you start with still water in the sink, it just goes straight down the drain, unless you've done something to disturb the flow (hmm...like pull the plug in the drain...not sure how you could fill a tub or sink, let the water get still and then open the drain without introducing turbulence from the drain plug). You can't go according to toilet flushes, because toilets start out with the water stream angled into the bowl to make it swirl on purpose to ensure everything that may have splashed up on the sides gets washed off during the flush.
  13. Apr 14, 2005 #12
    I did specifically say not toilets. They're pretty obviously designed to swirl one way or the other.

    But it seems clear that at least as far as sinks are concerned (as long as they are sufficiently symmetrical) the members of the "no-swirlie" school are correct. In a series of meticulously-executed trials in which the water was allowed to remain in the sink long enough for all visible currents and perturbations to disappear before the drain was slowly and gently opened (by lever, from beneath), the water went down the drain without swirling. Coriolis effect, if any, was not observable.

    You can bet your bottoms on it.

    [It's amazing, the extent to which I'll go to avoid doing my homework. ]
    :rofl: :rofl: :rofl:
  14. Apr 14, 2005 #13
    Did you use any dyes? Maybe there was a current but seeing water moving in water can be difficult... :uhh:
    If there is no swishy vortex then it might be hard to tell if the water is actually moviong or not.
  15. Apr 14, 2005 #14
    My sink is not perfectly clean. :uhh:

    (Thus, there always seemed to be a small amount of "foreign matter"; sufficient to reveal the motion of the water in which it floated to a reasonable degree of certainty.) :biggrin:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook