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Why with water on the friction coeffiction is bigger?

  1. Mar 16, 2008 #1
    why with water on the friction coeffiction is bigger???

    on the corridor floor the static friction is 2.2N, the dynamic friction is 1.4N
    when the corridor floor with water on, the friction is increased, the static friction is 3.8N, the dynamic friction is 3.6N

    why with water on, the friction coeffiction is bigger?

    and, why with water on the difference between static friction and dynamic friction is decreased?

    water can't be a lubricator, but oil can. that is because the surface tension and the molecular structure on them. but what is their difference, and why it can cause the difference on the friction coefficient?

    thank u
  2. jcsd
  3. Mar 16, 2008 #2
    Water can act as an adhesive, probably due to the viscous drag of a thin film of water.
  4. Mar 16, 2008 #3


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    Welcome to PF!

    Hi Orange! Welcome to PF! :smile:

    I think you're thinking of aquaplaning … when you slip on a wet floor, you're essentially water-skiing rather than sliding on the floor itself.

    Wet cloth is definitely stickier than dry cloth.

    And whatever is sliding on the floor (like a mop) may absorb some of the water, which will give it different frictional properties anyway. :smile:
  5. Mar 16, 2008 #4


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    Try riding a bicycle on that wet floor and you'll find the coefficient of friction is smaller than with a dry floor. It depends on the two surfaces involved plus the water.
  6. Mar 16, 2008 #5


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    … academic terrorism ? …

    Don't let the janitor catch you! :redface:

    People have been expelled for less … :smile:
  7. Mar 16, 2008 #6
    As mentioned earlier, whether water is a lubricant or not depends on the actual surfaces. For, example, it lubricates PEEK pretty well.

    Our ordinary experience with water as a lubricant is skewed by impurities in the water which often dramatically change the lubrication properties. For example, add just a small bit of glycerine to water and check the coefficients.
  8. Mar 17, 2008 #7
    sorry, i know glycerine is a surface active agent, it can decrease the surface tension of the water
    but i don't know what is the relations between surface tension and friction
  9. Mar 17, 2008 #8
    So, pour a cup of water on a tile floor and walk across it. Then repeat, adding a tablespoon of glycerine to the water. Let us know what you find.
  10. Mar 18, 2008 #9
    It depends on how much water there is between the two surfaces. If water is abundant, it will act as a lubricant because the two surfaces do not slide on each other but on a thin layer of water. But if water is just enough, it will increase the friction for a number of reasons. One of them is that the water seals some possible cavities and creates vacuum which certainly increases the force perpendicular to the surfaces. Others may be adhesion or the likes.
  11. Mar 18, 2008 #10
    some of the atmospheric pressure, right?
  12. Mar 19, 2008 #11
    Excuse me. I think I missed something in your post. You said "corridor". Is this a polished floor? If so, you will probably see significant stiction.
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