A wire forming a loop is dipped into soap solution and taken out so that a film of soap solution is formed. A loop of 6.28 cm long thread is gently put on the film and the film is pricked with a needle inside the loop. The thread loop takes the shape of a circle. Find the tension in the thread. Surface tension of soap solution = 0.030 N m-1.
3 × 10-4 N
After pricking, the loop will take a circular shape as shown.
Hence, the total force is given by 
.
Surface tension of soap solution, 
Given, length of the thread, 
Hence, 
Let 
be the length of contact surface.
We know, 
From the figure, 
.
Hence,
For small angles, 
1