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Old Feb 10th 2016, 10:34 PM   #1
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force of bubble

why the force of bubble is given by the formula of 2(2 pi R ) σs ? why not (2 pi R ) σs + (2 pi R+ ΔR) σs ? Since the radius of th outer circumfernece and inner circumference are the different ?
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Old Feb 11th 2016, 07:44 AM   #2
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Because ΔR is assumed negligible.

The concept of a quantity being negligible is one you have tripped over several times now.
While your equation might be more "correct", the difference represented by the two different equations is tiny.
You would have to extend the display on your calculator several tens of times to find any difference between the results of the two alternative equations.
In other words the difference is too small to be worth worrying about,
Also leaving it out of the equation makes the calculation easier...

I suppose that strictly speaking the author of the text should have explicitly stated the assumption that ΔR is assumed negligible.
I guess it was just assumed to be so obviously negligible, that an explicit statement to that effect would be superfluous.
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