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Homework Help: Surface Tension

  1. Jan 8, 2009 #1
    I have been studying surface tension from my textbook, when I came across a question in which a thin circular plate is placed over water. The surface tension of the water is given. The minimum force required to pull away the circular plate is asked. The dilemma, I am in is in which direction will the water molecules pull the plate when it is dragged on the surface?

    All I have read about surface tension is that the direction is along the surface of fluid and perpendicular to th plate periphery. But there are two possibilities in that plane for force acting on each small element. I tried to google it but either google nor wiki mention anything about the specific direction.

    In my opinion th net force due to surface tension will be zero.
    But what I said is just a guess.!
  2. jcsd
  3. Jan 8, 2009 #2


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    I would think that they are asking for the force required to pull the plate straight up.
  4. Jan 8, 2009 #3
    The question goes as this.

    And the solution provided to this example is:
    Length of surface=2*pi*r
    Force required=T*l

    Now if the disc is being pulled up, why isn't the gravitational force playing any role??
  5. Jan 9, 2009 #4
    Well im not 100% Sure on this, probably not even 80%, but to me i would see it as gravity is playing a role in this problem. When you factor in the surface tension, arent you kind of already factoring in gravity? Because when you pull up, you will get an opposite force in the other direction which is caused by the surface tension (which part of the surface tension is caused by gravity??) So isnt gravity one component of surface tension that is already factored into the surface tension?
  6. Jan 9, 2009 #5
    Please help me knowing 2 things:
    1. The direction of the force because of surface tension.
    2. Why has the weight been neglected?
  7. Jan 10, 2009 #6

    (1.) Well, if you are lifting the plate upwards you will get an opposite pull in the downward direction (the same direction as g) which is the force of surface tension. So you have your force of the upward pull balanced by the surface tension force downward.

    (2.) As i said before, im not 100% but from my point of view i would see it as weight is already factored into surface tension. Surface tension is the molecules being pulled back down to the other molecules which it is connected to. Keep in mind those molecules are also being acted on by gravity simultaneously.
  8. Jan 13, 2009 #7
    Well, whats the reason for the downward direction of the force?

    Since NBAJam is not 100% sure about this, I would request some other member from PF to please guide me.
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