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,
Let be the length of contact surface.
From the figure, .
For small angles,
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