Q: A wire is bent in the form of a U – shape and a slider of negligible mass is connecting the two vertical sides of the U-shape. This arrangement is dipped in a soap solution and lifted, a thin soap film is formed in the frame. It supports a weight of 2.0 × 10-2 N . If the length of the slider is 40 cm, what is the surface tension of the film ?
Sol: W = 2.0 × 10-2 N
l = 40 cm = 0.4m
Upward force due to surface tension = T × 2l
In equilibrium, W = T × 2l
$\large T = \frac{W}{2l} = \frac{2 \times 10^{-2}}{2 \times 0.4} $
= 2.5 × 10-2 N/m