Question

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 the the thread. Surface tension of soap solution = 0.030 N m−1.

Answer 1

Tension in Thread is 3.0 x 1/10000 N

Explanation:

The Loop will take circular shape after pricking.

• Radius of which is given by the relation.

$2Tsin(d\theta)$ Force in inward direction is balanced by surface tension force in outward direction.

∴ $2T sin(d\theta) = (Surface\ Tension) \times (Length\ of\ arc)$

For small angles, $sin (d\theta) = d\theta$

∴ $2Td\theta = S(2Rd\theta)$

∴ $T = SR = (0.030)(10^-^2)$

= $3.0 \times 10^-^4 N$