Editing Surface tension (section)

==== In terms of force ====
Surface tension {{mvar|γ}} of a liquid is the force per unit length. In the illustration on the right, the rectangular frame, composed of three unmovable sides (black) that form a "U" shape, and a fourth movable side (blue) that can slide to the right. Surface tension will pull the blue bar to the left; the force {{mvar|F}} required to hold the movable side is proportional to the length {{mvar|L}} of the immobile side. Thus the ratio {{math|{{sfrac|''F''|''L''}}}} depends only on the intrinsic properties of the liquid (composition, temperature, etc.), not on its geometry. For example, if the frame had a more complicated shape, the ratio {{math|{{sfrac|''F''|''L''}}}}, with {{mvar|L}} the length of the movable side and {{mvar|F}} the force required to stop it from sliding, is found to be the same for all shapes. We therefore define the surface tension as

<math display="block">\gamma=\frac{F}{2L}.</math>
The reason for the {{sfrac|1|2}} is that the film has two sides (two surfaces), each of which contributes equally to the force; so the force contributed by a single side is {{math|''γL'' {{=}} {{sfrac|''F''|2}}}}.