A loop of 6.28 cm long thread is put gently on a soap film in a wire loop. The film is pricked with a needle inside the soap film enclosed by the thread. If the surface tension of soap solution is 0.030 N/m, then the tension in the thread is

When the soap film enclosed by the thread is pricked, the thread would take up a circular shape. Consider a small element of the thread. While horizontal components of tension get balanced the vertical components get added up.

$\begin{array}{l}\mathrm{or} 2\mathrm{T\theta }=2\mathrm{\sigma }(2\mathrm{\theta }\times \mathrm{r})\\ \mathrm{or} 2\mathrm{Tsin}\mathrm{\theta }=\sigma 2\mathrm{\Delta l}\\ \mathrm{or} \mathrm{T}=2\mathrm{\sigma r}=2\mathrm{\sigma }\times \frac{2\mathrm{\pi r}}{2\mathrm{\pi }}=2\times 0.030\times \frac{6.28\times {10}^{-12}}{2\mathrm{\pi }}\mathrm{N}\end{array}$
=6x10^-4 N