Surface tension trivial problem

  • #1
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 lenght 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?
 

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  • #2
olivermsun
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Do the forces on the two sides of the bar act in the same direction or do they push in opposite directions?
 
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  • #3
haruspex
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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?
 
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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
 

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