There is a horizontal film of soap solution. On it a thread is placed in the form of a loop. The film is pierced inside the loop and the thread becomes a circular loop of radius R. If the surface tension of the loop be T, then what will be the tension in the thread?
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When the film inside the loop is pierced, the surface tension acts along the circumference of the loop. The force due to surface tension is T per unit length. For a circular loop of radius R, the total length is the circumference, . The tension in the thread must balance this force. Therefore, the tension is equal to the total force from surface tension, which is .
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