If you mount an experiment like the one in the following visual you can calculate the surface tension of a soapy liquid. The mathematical expression for S.T. is obtained by dividing the force ( F ) exerted on the surface by the length over which it acts.
As the liquid surface has two faces, each face adheres to the upper part of the  frame over a distance  L.
The expression of the S.T. comes from sharing out the force over the contact length:

To find out the characteristics of surface tension click here.

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