# Surface tension trivial problem

• Klaus von Faust

#### Klaus von Faust

1. The films of the two liquids are separated by a bar of length l. The coefficients of surface tension of liquids are equal to s1 and s2, respectively. What force acts on the bar on the liquid side?(It is a rectangular surface of 2 liquids separated by a bar of length l)

2. Force=(surface tension coefficient )*lenght

3. I thought that the force which acts on the bar from one side is (surface tension coefficient)*lenght of the bar, but then the result will be the sum of 2 forces, F=l(s1+s2), the answer is wrong according to the book. Where is my mistake?

Do the forces on the two sides of the bar act in the same direction or do they push in opposite directions?

Klaus von Faust
Klaus von Faust said:
the force which acts on the bar from one side is (surface tension coefficient)*lenght of the bar
In addition @olivermsun 's hint, these are films. How many surfaces does a film have?

Klaus von Faust
haruspex said:
In addition @olivermsun 's hint, these are films. How many surfaces does a film have?
They have 2 surfaces. And the forces are pushing in opposite directions. That means F=2L|s1-s2|. Thank you