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Force of surface tension quandry?

  1. Oct 15, 2014 #1
    I have went through the thread and looked at the explanations for surface tension, and I have also went online to about 10 different websites, but I have not yet been able to figure out this concept. I think it is just my inability to understand and I need perhaps a different way of explaining.
    I am trying to see how the force of surface tension is pointing
    Here is what I know.


    Fw is the weight of the object, and Fs is parallel at the point where it contacts the object which is the force due to surface tension.

    Also, the water wants to stay at it's most energetically favorable state. And a sphere will allow minimum surface area for a given volume.

    Given all this information, I think the answer to my question is, 1. due to the net inward force, the outer water molecules, e.g. in space, are brought inward and into a sphere in which the outerwater molecules are more tightly bound in an inward fashion. Therefore, due to the fact that water is incompressible, there is a pressure inside the water molecule sphere and the pressure is also radiating outward.(I'm not sure if this pressure idea is right)

    2. The energy wants to keep it at this nice mode, therefore if say an object like a needle sits on the water, the water wants to maintain the energy that it was at.

    Please, if anyone can assist, I would be extremely thankful!!!!

  2. jcsd
  3. Oct 15, 2014 #2
    I like to think of surface tension as the same as that of a stretched membrane acting along the free surface. In the case of a spherical drop, it is like a balloon that is stretched around the drop. The surface tension is the force per unit length within the surface of the balloon. Unlike an ordinary balloon, however, no matter how much the surface area increases, the force per unit length doesn't change. In the case of the needle, the surface tension is the force per unit length along the line of contact between the fluid free surface and the needle. The surface tension always acts in the direction tangent to the free surface. Hope this helps.

  4. Oct 16, 2014 #3
    Hi Sir Chet,

    From your explanation I feel very close to understanding. I just have two more questions: I was wondering if there was no needle, if the direction of force per unit length would be horizontal with the neighboring outer water molecules for the outer most water layer for ex. a ball of water in space. Lastly, is the force due to the hydrogen bonds of neighboring atom?

    Thank you Chet!!!
  5. Oct 16, 2014 #4
    I don't understand this question, particularly the part about the ball.

    On a flat horizontal free-surface, the surface tension acts horizontally within the surface.
    I'm not knowledgeable about the molecular explanation. What I do know is how to model surface tension effects.

  6. Oct 16, 2014 #5
    Ahh I think I get the satisfying gist of the concept! Thank you so much Chet for your benevolent help!
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