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Surface Tension Force

  1. Jan 20, 2007 #1
    Hi! I'm revising for my graduation exams and I unfortunately got stuck thinking about surface tension, esp about the example of a needle lying on water. When you look on this drawing from Wikipedia, only two surface tension forces are shown. These two forces are tangent lines at the points where the needle and water surface get separated. But what about the other points? Are there any surface tension forces at the red points in the next drawing? Or do surface forces act (exist) only along "seperation" lines? Thanks!
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  3. Jan 20, 2007 #2
    And the same problem here (PDF, 164KB). In Example 1, "Floating a Needle on the Surface of Water", the weight of the heaviest needle that would still float on the surface is calculated. And again, only two surface tension forces are being taken into consideration..Why?
  4. Jan 20, 2007 #3


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    There is no NET force in those reddened points. The infinitesimal thickness of the layer water in contact with the solid is in internal equilibrium. If you slice the film on those points the net force is zero. It's pretty similar to what happens in a middle section of a loaded beam. Even though there are internal stresses the stress distribution is such that elastic equilibrium holds in each section of the beam. On those places on which there is no force exerted by the solid the internal equilibrium must hold too, and the internal surface tension force is cancelled with the hydrostatic pressure, that's why you see curvature even on zones that are not in direct contact with the solid. Imagine the layer of fluid as a belt that surrounds the needle and has some elastic (in this case surface tension) energy stored on it because of the action of the solid over it.
  5. Jan 20, 2007 #4
    thanks for your reply, but unfortunately i don't really get it... could you elaborate on that a little?
  6. Jan 20, 2007 #5


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    When you want to find the net force on a part of your system, yo do not have to worry about the various forces internal to that part. For instance, to find the tension in a length of string, you consider the string (or some part of it) as your system and look at the forces acting on its ends. That there are "similar" forces acting at every point inside this selected system is inconsequential to the determination of the net force acting on the system.

    Likewise with surface tension.
  7. Jan 21, 2007 #6
    Still a little bit confused. I know what you mean by the loaded timber, but what exactly attracts the molecules in the surface layer together, so they don't draw aside under the needle?

    In textbooks, when illustrating the surface tension force, an example with a wire frame and a wire slider is used. But that's - in my opinion - is quite different from the example with a needle since the wire slider is on the edge of the surface, not lying ON the surface.
  8. Jan 21, 2007 #7


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    As far as I remember the superficial layer of water does not break down due to an disbalance in the intermolecular forces in the molecules that are lying on the surface. That intermolecular force acts on let's say the coordinate 's' which represents a tangential coordinate to the water surface, in a similar manner that tension force acts perpendicular to every section of the string mentioned by gokul.
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