First slide

Pressure in a fluid-mechanical properties of fluids

Question

A thin square plate of side 5 cm is suspended vertically from a balance so that lower side just dips into water with side to surface. When the plate is clean ( $θ$ :0^o), it appears to weigh 0.044 N. But when the plate is greasy ( $θ$ =180^o), it appears to weigh 0.03 N. The surface tension of water is

Moderate

A

3.5 x 10^-2 N/m
B

7.0 x 10^-2 N/m
C

14.0 x 10^-2 N/m
D

1.08 N/m

Solution

Difference in apparent weights is due to differences in forces of surface tension. Due to 180^o, the force surface tension in one case is opposite to the force of surface tension in the other case.
$\begin{array}{l} ∴ 2 \times σ_{w} \times \frac{10}{100} = 0.004 - 0.03 \\ or σ_{w} \times \frac{0.014}{1} \times 5 N / m = 0.07 N / m = 7 \times 10^{- 2} N / m \end{array}$

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