1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Strange fluid-mechanics formula made by crazy textbook author

  1. Aug 27, 2013 #1
    Please look at the picture I uploaded.

    Can somebody explain to me how the author of my book can claim that the "vertical component of the ring surface-tension force at the interface in the tube balances the weight of the column of fluid of height h"? This stuff is weird. How can TENSION FORCES which are on the TOP of a column of fluid support the weight of the column?

    I mean, it's obviously the pressure-difference between the air inside the tube and outside which lifts the water, assuming that the tube is closed on the top.

    Attached Files:

    Last edited by a moderator: Aug 27, 2013
  2. jcsd
  3. Aug 27, 2013 #2
    The tube need not be closed at the top. The air pressure is equal at the top of the fluid in the tube and at its base outside. So there must be something else that counteracts the weight of the fluid in the tube. This is what the author states.
  4. Aug 27, 2013 #3
    Then how does it work? Does the fluid actually "stick" to the sides of the tubes, and thus drags some of the rest of the fluid along with it? So, if you lift the tube very carefully, theoretically the water would stick to it?

    How can the fluid, say H2O, stick to a normal glass or plastic tube? I mean neither of those substances are polarized...

    And can somebody pls help me here too? https://www.physicsforums.com/showthread.php?p=4484367#post4484367
  5. Aug 27, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

  6. Aug 27, 2013 #5
    Hmm, thanks. But 2 questions remain:

    1) Why is the surface-tension connecting the water to the tube-wall the same as the regular surface-tension?

    2) Doesn't this mean that there will be a tension vertically throughout the water, which lifts the water up the tube?
  7. Aug 27, 2013 #6
    Given how the solution is framed in your book, I would expect that it should have an explanation of the mechanism before it plunges into the calculation. Is it completely silent on that?
  8. Aug 27, 2013 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    1. surface tension is surface tension. It's the fluid and the size (diameter) of the tube which produces this phenomenon.

    2. At least in the tube.

    Weird corollary to capillary action. When the fluid is mercury, Cap Action produces a depression in the fluid level relative to the level of the rest of the liquid.
  9. Aug 27, 2013 #8
    The book we use is called Fluid Mechanics by Frank M. White, and it is only fit as firewood.

    Anyway I have understood this subject thanks to you guys and Cengel & Cimbala's far superior text on fluid mechanics. thanks :)
    Last edited: Aug 27, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook