A drop of water is placed between 2 parallel glass plates which are then pressed together until the

Question

A drop of water is placed between 2 parallel glass plates which are then pressed together until the water film is 0.01 cm thick. If the area of water in contact with the plates is $10 \mathrm{sq} . \mathrm{cm}$. what is the force needed to separate them pulling normally? S.T. of water is 75 dyne/cm\

Professor PC Thomas Classes · Answer

When a drop of liquid is placed on a glass plate and another glass plate is placed over it, an approximately circular area of the glass plate will be wet by the liquid. Let A be the area of the wet portion and d the distance between the plates. Then one plate can easily slide over the other. But a force $F=\frac{2 A T}{d}$ will be required to separate them by pulling the plates apart\ $\mathrm{F}=\frac{2 \mathrm{AT}}{\mathrm{d}}$\ A $=10 \mathrm{~cm}$\ T = 75 dyne/cm\ $\mathrm{d}=0.1 \mathrm{~cm}$\ F will be obtained in dynes\ 980 dynes $=1 \mathrm{~g}$ wt.\