A soap film of surface tension T is formed on a rectangular wire frame. A small closed loop made from a thread of length ℓ is placed irregularly on the film. A hole is pricked in the film inside the loop and the thread reaches equilibrium. Choose the correct option(s). Image
Select all that apply

Question

A soap film of surface tension T is formed on a rectangular wire frame. A small closed loop made from a thread of length ℓ is placed irregularly on the film. A hole is pricked in the film inside the loop and the thread reaches equilibrium. Choose the correct option(s). Image

Select all that apply

G1 Physics · Accepted Answer

Step 1: After the film inside the loop is removed, surface tension acts normally outward all along the thread. A closed curve under uniform normal pull has maximum enclosed area when it is a circle. Step 2: For a circular loop, radius R = ℓ/(2π). Force balance on a semicircle gives 2F = 2T(2R), so F = 2TR = Tℓ/π. Correct answers: B, D.