Consider a bubble of soap solution. Find the surface tension force between the left half surface and right half surface

Explanation

The bubble also have two surfaces: the inner surface and the ouyer surface. And in the small thickness between them, there is some liquid. So the surface tension force will be applied by inner surface as well as the outer `[T(2piR)]`.
So total tension force between left half and right half surface is
`F = (T)(2piR) + (T)(2piR) = T(4piR)`