Q: A small piece of wire of length 4 cm is floating on the surface of water. If a force of 560 dyne in excess of its apparent weight is required to pull it up from the surface, find the surface tension of water.

Sol: Length of wire l = 4 cm.

Contact length of solid with liquid L = 2l = 8 cm.

Surface tension $\large T = \frac{F}{L} = \frac{560}{8}$

T = 70 dyne/cm

T_{water} = 70 dyne/cm = 0.07 N/m