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 `20cm` and its weight is `0.1N`
A. `0.125N`
B. `0.225N`
C. `0.115N`
D. `0.130N`

Answer 1

Correct Answer - D
The thin ring is in contact with water from both inside and outside. So, contact length is `2xx20=40cm`
`F_(min)=F_(ST)+W=(75xx10^-5)xx40+0.1=0.130N`