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**1.**Two coaxial rings of radius R=10 cm are placed to a distance L from each other.There is a soap film connecting the two rings(that looks like a cylinder which have different radii with z coordinate. (The rings lie in xy plane)).Derive a differential equation describing the shape r(z) of the film where r is the radial distance from symmetry axis,as a function of distance z along the axis.

Show that cosh x is one of its solutions.

When the distance between rings is slowly increased ,at a certain critical distance L• ,the soap film breaks.Find L•

**2.**surface tension T=F/l and

Excess pressure inside a soap bubble is ∆P=4T/r

**3.**I write the force balance equation for a divided part of soap film.

T.4Πr=∆P.Πr^2

and then

∆p=4T/r but I can't move ahead .Long hints is requested.

Thanks in advance.

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