Question

A thread is tied slightly loose to a wire frame as in figure and the frame is diped into a soap solution and taken out. The frame is completely covered with the film. When the portion A is punctured with a pin, the thread :

• A. becomes concave towards A
• B. becomes convex towards A
• C. either (a) or (b) depending on the size of A with respect to B
• D. remains in the initial position

Answer 1

The correct option is A becomes concave towards A

The potential energy of the surface tension force is given by the formula

$U=SA$ where $A$ is the area of the film. Since after puncturing, the film will try to minimize its potential energy and hence the surface area, the thread will become concave towards $A$.