First slide

Surface tension

Question

The surface tension of water is 75 dyne/cm. Find the minimum vertical force required to pull a thin wire ring up (refer figure) if it is initially resting on a horizontal water surface. The circumference of the ring is 20 cm and its weight is 0.1 N

Moderate

A

0.125 N
B

0.225 N
C

0.115 N
D

0.130 N

Solution

The thin ring is in contact with water from both inside and outside. So, contact length is 2x20 = 40 cm
$F_{\min} = F_{ST} + W = (75 \times 10^{- 5}) \times 40 + 0.1 = 0.130 N$

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