Hello ,(adsbygoogle = window.adsbygoogle || []).push({});

A Problem I found In a book , although I have the answer , I cant understand it

the problem is

Calculate the force required to separate two glass plates of area 10 cm^2 each , with a film of water .5mm thick between them . take the coefficent of surface tention (T)to be 72 dyne/cm and theta = 0

The Answer written

since theta = 0 so the radius of curvature of the cylindrical surface between the plates is r = d/2 =.025 cm

((note : I Understand this but what If theta was another angle ??))

Delta P = T/r = 72/.025 = 2880 dyne/cm^2

the Normal force required = Delta P * Area = 2880*10 =2.88*10^4 dyne = .288 N #

I nead some explanation for this

Another _Identical problem_ says

two circular plates of radus 5 cm with 5 mm thick film of water betweem them , If the surface tention = 72.7*10^3 N/m , the normal force required to pull them Apart is what ?

it is the same I know , but in the solution ((the solution this time is not in the book , it is an assistant Engineer at the college)) said that :

Delta P = T(1/R1-1/R2) , R2 = infinity , so Delta P = T/R1

My problem is Why did he write a negative sign ?? I know it won't change the answer but why is it minus or it is just a writing error ??

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Surface tention problem

**Physics Forums | Science Articles, Homework Help, Discussion**