Question

A rectangular frame of $7cm\times 5cm$ hangs vertically by an arm of a balance. When its edge is just dipped in soap solution in container, extra mass of $0.4g$ must be placed in opposite arm to balance the pull of the film in order to just raise it. Determine surface tension of soap solution in dyne/cm.

Answer 1

Answer: 28.00

A soap film is formed between edge (length) of frame and soap solution in container.
$\therefore T=\frac{mg}{2l}=\frac{0.4\times 980}{2\times 7}$
$=28dyne/cm$