Question

A thin metal disc of radius `r` floats on water surface and bends the surface downwards along the perimeter making an angle `theta` with vertical edge of the disc. If the disc displaces a weight of water `W` and surface tension of water is `T`, then the weight of metal disc is :
A. `2pirT+W`
B. `2pirTcos theta-W`
C. `2pirTcos theta+W`
D. `W-2pirTcos theta`

Answer 1

Correct Answer - C
Weight of metal disc = total upward force
= upthrust force + force due to surface tension
= weight of displaced water `+ T cos theta (2pir)`
`W+2pirTcos theta`